Re: centralize CPU feature detection

[Zsolt Parragi]

Both look good to me.

This isn't part of the patch, and it seems harmless, but while
reviewing the CRC functions, I noticed that pg_crc32c.h  is
inconsistent with its dllimport markers, pg_comp_crc32c has 3
different declarations, and only 1 of them is marked PGDLLIMPORT.