[libcxx] Temporarily switch back to pthread backend for Fuchsia

We switched to C11 thread API on Fuchsia in ab9aefe, but further
testing showed that Fuchsia's C11 mutex implementation needs a few
improvements for this to be usable, so we temporarily switch back
to the pthread implementation until those issues are addressed.

Differential Revision: https://reviews.llvm.org/D72862

libcxx/include/__config

@@ -1125,7 +1125,8 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
       (defined(__MINGW32__) && __has_include(<pthread.h>))
 #    define _LIBCPP_HAS_THREAD_API_PTHREAD
 #  elif defined(__Fuchsia__)
-#    define _LIBCPP_HAS_THREAD_API_C11
+     // TODO(44575): Switch to C11 thread API when possible.
+#    define _LIBCPP_HAS_THREAD_API_PTHREAD
 #  elif defined(_LIBCPP_WIN32API)
 #    define _LIBCPP_HAS_THREAD_API_WIN32
 #  else