[libc++] Refactoring __sync_* builtins; NFC

Summary: Wrap __sync_* builtins with __libcpp_ functions to facility future customizations as atomic operations are unavailable on some targets.

Reviewers: danalbert, EricWF, jroelofs

Subscribers: joerg, llvm-commits

Differential Revision: https://reviews.llvm.org/D34918

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307591 91177308-0d34-0410-b5e6-96231b3b80d8

src/locale.cpp

@@ -667,7 +667,7 @@ locale::id::__get()
 void
 locale::id::__init()
 {
-    __id_ = __sync_add_and_fetch(&__next_id, 1);
+    __id_ = __libcpp_sync_add_and_fetch(&__next_id, 1);
 }
 
 // template <> class collate_byname<char>