Use C11 alignas instead of pg_attribute_aligned

Use C11 alignas instead of pg_attribute_aligned

Replace pg_attribute_aligned with C11 alignas, for consistency with
current conventions.

(These new uses were added by commit fbc57f2bc2e, which was developed
concurrently with the switch from pg_attribute_aligned to C11 standard
alignas, and it ended up being committed with the "old" style.)

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/CANWCAZaKhE+RD5KKouUFoxx1EbUNrNhcduM1VQ=DkSDadNEFng@mail.gmail.com