[libc++] Fix native wait alignment (#180928)
This PR fixes two issues regarding the alignment of native wait:
- In the internal platform call, the local variable is copied from a
potentially non-aligned buffer
- Under the unstable ABI, the predicate to test eligibility of a type
being able to do native wait is purely on size. We should test also the
alignment of such type is qualified for platform call
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
