libstdc++: Correct requirements for atomic/cons/zero_padding.cc tests [PR124124].

libstdc++: Correct requirements for atomic/cons/zero_padding.cc tests [PR124124].

This test uses atomic<T>::compare_exchange_strong and thus may require
libatomic and atomic-cmpxchg-word. The requirement on thread-fence and
non_pch were superfulous.

	PR libstdc++/124124

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic/cons/zero_padding.cc: Adjust
	dg-require.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>