linuxkpi: Add asm/set_memory.h

Provide functions needed for drm-kmod.

MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35571

(cherry picked from commit 0e45856f9a2d4f136cbfb4091ad254d0461f4614)