Emulate clang atomic built-ins on gcc > 4.7

gcc 4.7 and above has atomic built-ins which slightly different APIs
from those provided by clang. Add proxy functions that wrap the gcc
built-ins to produce a symbol that is API equivalent to the clang
built-ins. This allows libc++'s atomic library to be used with gcc-4.7
and newer.

Patch contributed by Albert Wong.