libstdc++: Clear padding bits in atomic compare_exchange

libstdc++: Clear padding bits in atomic compare_exchange

This change implements P0528 which requires that padding bits not
participate in atomic compare exchange operations. All arguments to the
generic template are 'sanitized' by the __builtin_clear_padding intrinsic
before they are used in comparisons. This requires that any stores
also sanitize the incoming value.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Thomas Rodgers <trodgers@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/atomic_base.h (__atomic_impl::__maybe_has_padding):
	New function.
	(__atomic_impl::clear_padding): Likewise.
	(__atomic_impl::__compare_exchange): Likewise.
	(__atomic_impl::compare_exchange_weak): Delegate to
	__compare_exchange.
	(__atomic_impl::compare_exchange_strong): Likewise.
	* include/std/atomic (atomic<T>::atomic(T)): Clear padding when
	possible in a constexpr function.
	(atomic::store): Clear padding.
	(atomic::exchange): Likewise.
	(atomic::compare_exchange_weak): Use __compare_exchange.
	(atomic::compare_exchange_strong): Likewise.
	* testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
	test.
	* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
	New test.