LinuxKPI: Add __diag macros

__diag macros turn individual warnings and errors on and off locally,
depending on version of compiler. Add dummy implementation as drm-kmod
set warnings separately for each file.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: manu, emaste
Differential Revision: https://reviews.freebsd.org/D45842

(cherry picked from commit 742088d9fb545296c8677119757a967141ba7d4a)