Use C11 alignas in pg_atomic_uint64 definitions

Use C11 alignas in pg_atomic_uint64 definitions

They were already using pg_attribute_aligned.  This replaces that with
alignas and moves that into the required syntactic position.  This
ends up making these three atomics implementations appear a bit more
consistent, but shouldn't change anything otherwise.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/46f05236-d4d4-4b4e-84d4-faa500f14691%40eisentraut.org