[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.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>