linuxkpi: Add asm/processor.h

Also fill the boot_cpu_data struct as drm needs it.

Reviewed by: bz
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36107

(cherry picked from commit b2c860060ccd6020f313f48107694ef207acfc9a)