C++20 deprecated ATOMIC_FLAG_INIT thinking it was deprecated in C when it
wasn't. It is expected to be undeprecated in C++23 as part of LWG3659
(https://wg21.link/LWG3659), which is currently Tentatively Ready.
This handles the case where the user includes <stdatomic.h> in C++ code in a
freestanding compile mode. The corollary libc++ changes are in
1544d1f9fdb115782202d72ad200c3f93b2c4f5a.
C17 deprecated ATOMIC_VAR_INIT with the resolution of DR 485. C++
followed suit when adopting P0883R2 for C++20, but additionally chose
to deprecate ATOMIC_FLAG_INIT at the same time despite the macro still
being required in C. This patch marks both macros as deprecated when
appropriate to do so.