builtins: Implement the functions from stdatomic.h

Talking to John and Doug, we concluded that these functions from
stdatomic really do belong here in compiler-rt rather than in libc,
since the compiler owns stdatomic.h and these need to refer to
clang-specific builtins. Nonetheless, I've only added these on darwin
for now - other platforms should probably do the same unless their
libc does implement these functions.

llvm-svn: 236805