_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