[libc++] Adapt a few things around the implementation of P1135R6

- Add the new symbols to the ABI list on Darwin
- Add XFAIL markup to the tests that require dylib support on older platforms
- Add availability markup for back-deployment
diff --git a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
index 943fc84..3283bf2 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <barrier>
 
 #include <barrier>
diff --git a/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
index 8724a1d..a0428a7 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive_and_drop.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <barrier>
 
 #include <barrier>
diff --git a/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp b/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
index 8596e8a..66f46ec 100644
--- a/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/arrive_and_wait.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <barrier>
 
 #include <barrier>
diff --git a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
index 67ae526..1cce370 100644
--- a/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
+++ b/libcxx/test/std/thread/thread.barrier/completion.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <barrier>
 
 #include <barrier>
diff --git a/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp b/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
index 156e913..07b3c76 100644
--- a/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/arrive_and_wait.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <latch>
 
 #include <latch>
diff --git a/libcxx/test/std/thread/thread.latch/count_down.pass.cpp b/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
index 81f0605..9374a6e 100644
--- a/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/count_down.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <latch>
 
 #include <latch>
diff --git a/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp b/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
index 7284849..0524271 100644
--- a/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
+++ b/libcxx/test/std/thread/thread.latch/try_wait.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <latch>
 
 #include <latch>
diff --git a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
index bb5f7fb..66e1375 100644
--- a/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/acquire.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <semaphore>
 
 #include <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
index d46d6ae..fc867be 100644
--- a/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/binary.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <semaphore>
 
 #include <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
index 0660feb..fe3c70a 100644
--- a/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/release.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <semaphore>
 
 #include <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
index d0bbc91..39badb0 100644
--- a/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/timed.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <semaphore>
 
 #include <semaphore>
diff --git a/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp b/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
index 7f06567..7c8ded0 100644
--- a/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
+++ b/libcxx/test/std/thread/thread.semaphore/try_acquire.pass.cpp
@@ -9,6 +9,15 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
+// This test requires the dylib support introduced in D68480
+// XFAIL: with_system_cxx_lib=macosx10.15
+// XFAIL: with_system_cxx_lib=macosx10.14
+// XFAIL: with_system_cxx_lib=macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+
 // <semaphore>
 
 #include <semaphore>