Adapt gcc 4.7's __atomic_ built-in to clang's __c11_* built-ins.

Someone really familiar with intrinsics and atomics should verify this code.
I think it is right, but I'm very scared of it.

Change-Id: I68f381b3186a559ab4545f7c2042d3042b898077
_LIBCPP_IS_LITERAL already abstracts feature + version check

Change-Id: I622ec334a5da6bfd974fcb27d3dbc01f0fa3d1e5
atomics: Add __. Diasllow math on arrays. Make wrappers static inline.

Change-Id: Iee72d1dd9b9d7fefe79d53a89b7aa09591e02f13
Add non-volatile versions for each wrapper.

Change-Id: I04ea13ebf44699bb9e64e73b62d29e25db57ef9b
Remove silly const_cast

Change-Id: Id68ae78b5b4baff9a213710466f1ddce1a497472
Merge pull request #136 from awong-dev/support-gcc47-atomics

Support gcc47 atomics