LinuxKPI: Add static_assert() macro

And move it along BUILD_BUG family in to
linux/build_bug.h to match Linux.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42800

(cherry picked from commit 31ef29091b0fbadefb4ccfb21e4b30ca7ddca3ed)