[libc++] Replace __sync_* functions with __libcpp_atomic_* functions

Summary:
This patch replaces __sync_* with __libcpp_atomic_* and adds a wrapper
function for __atomic_exchange to support _LIBCPP_HAS_NO_THREADS.

Reviewers: EricWF, jroelofs, mclow.lists, compnerd

Reviewed By: EricWF, compnerd

Subscribers: compnerd, efriedma, cfe-commits, joerg, llvm-commits

Differential Revision: https://reviews.llvm.org/D35235

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@313694 91177308-0d34-0410-b5e6-96231b3b80d8

src/locale.cpp

@@ -36,6 +36,7 @@
 #endif
 #include <stdlib.h>
 #include <stdio.h>
+#include "include/atomic_support.h"
 #include "__undef_macros"
 
 // On Linux, wint_t and wchar_t have different signed-ness, and this causes
@@ -667,7 +668,7 @@ locale::id::__get()
 void
 locale::id::__init()
 {
-    __id_ = __sync_add_and_fetch(&__next_id, 1);
+    __id_ = __libcpp_atomic_add(&__next_id, 1);
 }
 
 // template <> class collate_byname<char>