[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/exception.cpp

@@ -31,6 +31,7 @@
 #include "support/runtime/exception_glibcxx.ipp"
 #include "support/runtime/exception_pointer_glibcxx.ipp"
 #else
+#include "include/atomic_support.h"
 #include "support/runtime/exception_fallback.ipp"
 #include "support/runtime/exception_pointer_unimplemented.ipp"
 #endif