In some cases #if was used instead of #ifdef in an inconsistent style.
Cleaning this up also helps when analyzing cases like
38d8dce6 where this makes a
difference.

There are no behavior changes here, but the change in pg_bswap.h would
prevent possible accidental misuse by third-party code.

Discussion: https://www.postgresql.org/message-id/flat/3b615ca5-c595-3f1d-fdf7-a429e564f614%402ndquadrant.com