libstdc++: Strip cv-qualifiers in std::atomic<FP> (PR 95282)

libstdc++: Strip cv-qualifiers in std::atomic<FP> (PR 95282)

This improves the previous fix for PR 95282, and extends it to also
apply to the exchange function (which has a similar problem and would
become ill-formed with my proposed fix for PR 95378).

	PR libstdc++/95282
	* include/bits/atomic_base.h (__atomic_impl::load): Use the _Val
	alias instead of deducing _Tp as an unqualified type.
	(__atomic_impl::exchange): Use the _Val alias to remove volatile
	from the reinterpret_cast result type.