linuxkpi: Add some memset functions
Needed by drm-kmod
Obtained from: OpenBSD
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35943
(cherry picked from commit fa1f02baafd3d46e9d09a3715be23e96b601d9c9)