loader: remove libsa/crc32.c and use version from zlib

we have crc32(const void *, size_t) in libsa. Unfortunately zlib has
crc32(long, const unigned char *, unsigned) and we have conflict.

Since we do build libsa with zlib, we can use zlib version instead.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D24068