PR c/65083 notes that some functions in <stdatomic.h> are normal
functions, not generic functions, and so need to have out-of-line
copies that can be called when macro expansion is suppressed (unlike
the generic functions where DR#419 makes it undefined if you suppress
a macro expansion).

This patch adds such out-of-line definitions in libatomic for those
six functions, at a new LIBATOMIC_1.2 symbol version, as trivial
wrappers to the <stdatomic.h> macros, along with declarations of those
functions in <stdatomic.h>.  Tests are added that are based on the
corresponding tests for the macros, but with parentheses around the
function names to force the out-of-line functions to be used.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc:
	* ginclude/stdatomic.h (atomic_thread_fence, atomic_signal_fence)
	(atomic_flag_test_and_set, atomic_flag_test_and_set_explicit)
	(atomic_flag_clear, atomic_flag_clear_explicit): Declare as
	f...