stand: Only compile decompression routines

We don't need the compress rotuines, nor zstd_opt.c. Remove them.
Expand the number of places we omit code for IN_LIBSA (which are FreeBSD
specific). Due to the agressive optimization, though, this doesn't
reduce the size of the loader. It does reduce the number of 'false
positives' for places to omit to reduce the size as well as reducing the
build time slightly.

Sponsored by: Netflix
Reviewed by: tsoome, delphij
Differential Revision: https://reviews.freebsd.org/D36145