[libcxx] Add atomic_support.h header to src that handles needed atomic operations.

Summary:
In some places in libc++ we need to use the `__atomic_*` builtins. This patch adds a header that provides access to those builtins in a uniform way from within the dylib source.

If the compiler building the dylib does not support these builtins then a warning is issued.

Only relaxed loads are needed within the headers. A singe function to do these relaxed loads has been added to `<memory>`.

This patch applies the new atomic builtins to `__shared_count` and `call_once`.

Reviewers: mclow.lists

Subscribers: majnemer, jroelofs, cfe-commits

Differential Revision: http://reviews.llvm.org/D10406

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@241532 91177308-0d34-0410-b5e6-96231b3b80d8

include/mutex

@@ -175,6 +175,7 @@ template<class Callable, class ...Args>
 #include <__config>
 #include <__mutex_base>
 #include <functional>
+#include <memory>
 #ifndef _LIBCPP_HAS_NO_VARIADICS
 #include <tuple>
 #endif
@@ -539,7 +540,7 @@ inline _LIBCPP_INLINE_VISIBILITY
 void
 call_once(once_flag& __flag, _Callable&& __func, _Args&&... __args)
 {
-    if (__flag.__state_ != ~0ul)
+    if (__libcpp_relaxed_load(&__flag.__state_) != ~0ul)
     {
         typedef tuple<_Callable&&, _Args&&...> _Gp;
         _Gp __f(_VSTD::forward<_Callable>(__func), _VSTD::forward<_Args>(__args)...);
@@ -555,7 +556,7 @@ inline _LIBCPP_INLINE_VISIBILITY
 void
 call_once(once_flag& __flag, _Callable& __func)
 {
-    if (__flag.__state_ != ~0ul)
+    if (__libcpp_relaxed_load(&__flag.__state_) != ~0ul)
     {
         __call_once_param<_Callable> __p(__func);
         __call_once(__flag.__state_, &__p, &__call_once_proxy<_Callable>);