stdatomic: Only use clang atomics if clang is defined

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

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D46001

(cherry picked from commit 680f40f38343de118d5b973129683804e496faaf)