[libc++][AIX] Initial patch to unblock the libc++ build on AIX

This path would unblock the build of libc++ library on AIX:
1. Add _AIX guard for _LIBCPP_HAS_THREAD_API_PTHREAD
2. Use uselocale to actually take the locale setting
   into account.
3. extract_mtime and extract_atime mod needed for AIX. As stat
   structure on AIX uses internal structure st_timespec to store
   time for binary compatibility reason. So we need to convert it
   back to timespec here.
4. Do not build cxa_thread_atexit.cpp for libcxxabi on AIX.

Differential Revision: https://reviews.llvm.org/D97558

GitOrigin-RevId: 158026301b4842bb877fc8a6a444c348da6d9f3b
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a2945f0..32a998e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -36,7 +36,8 @@
   )
 endif()
 
-if (LIBCXXABI_ENABLE_THREADS AND (UNIX OR FUCHSIA) AND NOT (APPLE OR CYGWIN))
+if (LIBCXXABI_ENABLE_THREADS AND (UNIX OR FUCHSIA) AND NOT (APPLE OR CYGWIN)
+    AND NOT (${CMAKE_SYSTEM_NAME} MATCHES "AIX"))
   list(APPEND LIBCXXABI_SOURCES
     cxa_thread_atexit.cpp
   )