[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/new.cpp

@@ -12,6 +12,7 @@
 #include <stdlib.h>
 
 #include "new"
+#include "include/atomic_support.h"
 
 #if defined(_LIBCPP_ABI_MICROSOFT)
 // nothing todo