libstdc++: Use a loop in atomic_ref::compare_exchange_strong [PR111077]

We need to use a loop in std::atomic_ref::compare_exchange_strong in
order to properly implement the C++20 requirement that padding bits do
not participate when checking the value for equality. The variable being
modified by a std::atomic_ref might have an initial value with non-zero
padding bits, so when the __atomic_compare_exchange built-in returns
false we need to check whether that was only because of non-equal
padding bits that are not part of the value representation. If the value
bits differ, it's just a failed compare-exchange. If the value bits are
the same, we need to retry the __atomic_compare_exchange using the value
that was just read by the previous failed call. As noted in the
comments, it's possible for that second try to also fail due to another
thread storing the same value but with differences in padding.

Because it's undefined to access a variable directly while it's held by
a std::atomic_ref, and because std::atomic_ref will only ever store
values with zeroed padding, we know that padding bits will never go from
zero to non-zero during the lifetime of a std::atomic_ref. They can only
go from an initial non-zero state to zero. This means the loop will
terminate, rather than looping indefinitely as padding bits flicker on
and off. In theory users could call __atomic_store etc. directly and
write a value with non-zero padding bits, but we don't need to support
that. Users doing that should ensure they do not write non-zero padding,
to be compatibile with our std::atomic_ref's invariants.

This isn't a problem for std::atomic<T>::compare_exchange_strong because
the initial value (and all later stores to the variable) are performed
by the library, so we ensure that stored values always have padding bits
cleared. That means we can simply clear the padding bits of the
'expected' value and we will be comparing two values with equal padding
bits. This means we don't need the loop for std::atomic, so update the
__atomic_impl::__compare_exchange function to take a bool parameter that
says whether it's being used by std::atomic_ref. If not, we can use a
simpler, non-looping implementation.

libstdc++-v3/ChangeLog:

	PR libstdc++/111077
	* include/bits/atomic_base.h (__atomic_impl::__compare_exchange):
	Add _AtomicRef non-type template parameter and use a loop if it
	is true.
	(__atomic_impl::compare_exchange_weak): Add _AtomicRef NTTP.
	(__atomic_impl::compare_exchange_strong): Likewise.
	(atomic_ref::compare_exchange_weak): Use true for NTTP.
	(atomic_ref::compare_exchange_strong): Use true for NTTP.
	* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
	Fix test to not rely on atomic_ref::load() to return an object
	with padding preserved.

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