LinuxKPI: extract stringify() in their own header file

Add linux/stringify.h as directly included by drivers. Remove the
definitions from compiler.h and include the new header in places
where the stringify macros are already used without linuxkpi.

I have adjusted the Copyright of the new file according to the commit
originaly adding the macros (99e690772adbcb78c9e8441bedcf1913d72ea72f).

Sposnored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30440

(cherry picked from commit 43b4c00643bacb9d1ba3aa1022648104bc7ff4a2)