LinuxKPI: compiler.h add three more defines

Add fallthrough, ____cacheline_aligned_in_smp, and smp_mb() to
linux/compiler.h.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30420

(cherry picked from commit da717031c9fd914343e19ba87f19c7a9a7006721)