Work around buggy alignas in older g++

Work around buggy alignas in older g++

Older g++ (<9.3) mishandle the alignas specifier (raise warnings that
the alignment is too large), but the more or less equivalent attribute
works.  So as a workaround, #define alignas to that attribute for
those versions.

see <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89357>

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/3119480.1769189606%40sss.pgh.pa.us