Fix for Darwin buildbot failure due to r337037

Duplicate __get_unsafe_stack_bottom instead of using an alias for
platforms that don't suppport it like Darwin.

llvm-svn: 337044
diff --git a/compiler-rt/lib/safestack/safestack.cc b/compiler-rt/lib/safestack/safestack.cc
index cc6d81b..8af9362 100644
--- a/compiler-rt/lib/safestack/safestack.cc
+++ b/compiler-rt/lib/safestack/safestack.cc
@@ -267,8 +267,9 @@
 }
 
 extern "C"
-    __attribute__((visibility("default"), alias("__get_unsafe_stack_bottom")))
-    void *__get_unsafe_stack_start();
+    __attribute__((visibility("default"))) void *__get_unsafe_stack_start() {
+  return unsafe_stack_start;
+}
 
 extern "C"
     __attribute__((visibility("default"))) void *__get_unsafe_stack_ptr() {