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

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

LinuxKPI: Constantly use _LINUXKPI_ prefix in include guards

MFC after: 1 week
Reviewed by: bz, emaste, hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33562

linuxkpi: Add gcd function

This compute the common greater divider
Taken from OpenBSD

Reviewed by: bz, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26674