sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

tests/sys/kern/crc32: Check for SSE4.2 before using it

This avoids a SIGILL when running these tests on QEMU (which
defaults to a basic amd64 CPU without SSE4.2).

This commit also tests the table-based implementations in addition to
testing the hw-accelerated crc32 versions.

Reviewed By: cem, kib, markj
Differential Revision: https://reviews.freebsd.org/D28395

Separate kernel crc32() implementation to its own header (gsb_crc32.h) and
rename the source to gsb_crc32.c.

This is a prerequisite of unifying kernel zlib instances.

PR: 229763
Submitted by: Yoshihiro Ota <ota at j.email.ne.jp>
Differential Revision: https://reviews.freebsd.org/D20193