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

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

The requirements introduced in previous patch r16-7548-g060d7c2a9c1fe1,
were not sufficient for types of size bigger than 64B (Ctor or long double),
as dg-add-options of libatomic, links libatomic only if it is required to
handle atomics of 64B types or pointers.

This patch addresses above, by reducing the size of Ctor struct to fit in
64 bytes, and moving long double test to separate file, that requires and
links with libatomic.

	PR libstdc++/124124

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic/cons/zero_padding.cc: Updated
	Ctor class and move test_floating to...
	* testsuite/29_atomics/atomic_float/zero_padding.cc: Extracted
	test_floating.

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