stdatomic: Only use clang atomics if __clang__ is defined

[jhb]

Summary

GCC 14 defines has_extension(c_atomic) but does not support
c11_atomic_*, so require clang for the CLANG_ATOMICS case.

[imp]

We likely don't need sync atomics given the scopeof where it would be needed... but im not sure.

[jhb]

I think as part of your cleanup of cdefs.h, we can probably drop the pre-GCC 4.7 path entirely. We already don't support versions of GCC that old in cdefs.h now I think?