Align PGPROC to cache line boundary

Align PGPROC to cache line boundary

On common architectures, the PGPROC struct happened to be a multiple
of 64 bytes on PG 18, but it's changed on 'master' since. There was
worry that changing the alignment might hurt performance, due to false
cacheline sharing across elements in the proc array. However, there
was no explicit alignment, so any alignment to cache lines was
accidental. Add explicit alignment to remove worry about false
sharing.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/3dd6f70c-b94d-4428-8e75-74a7136396be@iki.fi