gcov: Fix build on AIX

We must guard used atomic builtins with GCOV_SUPPORTS_ATOMIC.
The patch is tested on AIX and I'm going to push it.

libgcc/ChangeLog:

	PR gcov-profile/95480
	* libgcov-profiler.c (GCOV_SUPPORTS_ATOMIC): Move to...
	* libgcov.h (GCOV_SUPPORTS_ATOMIC): ...here.
	(gcov_counter_add): Use GCOV_SUPPORTS_ATOMIC guard.
	(gcov_counter_set_if_null): Likewise.