C11 alignas instead of unions -- extended alignments

C11 alignas instead of unions -- extended alignments

This replaces some uses of pg_attribute_aligned() with the standard
alignas() for cases where extended alignment (larger than max_align_t)
is required.

This patch stipulates that all supported compilers must support
alignments up to PG_IO_ALIGN_SIZE, but that seems pretty likely.

We can then also desupport the case where direct I/O is disabled
because pg_attribute_aligned is not supported.

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