MaxAtomicPromoteWidth is defined as "the maximum width lock-free atomic
operation which will ever be supported for the given target", so an
oversized __c11_atomic_is_lock_free() or __atomic_is_lock_free() can be
evaluated to 0. This is advantageous in some scenarios (e.g. FreeBSD
powerpc, see D71600) to avoid the dependency on libatomic.
The behavior of __atomic_is_lock_free() will diverge from GCC as GCC
never evaluates it to 0 (gcc/builtins.c:fold_builtin_atomic_always_lock_free).