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 (99e690772a).
Sposnored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30440
