cpu_machdep.9: New manpage describing the semantics of several cpu_*

This page is not exhaustive but covers all of the MD interface
functions currently declared in <sys/proc.h>.

Requested by: kib
Reviewed by: kib
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D48022

(cherry picked from commit 9c87cbbcaaedbc4e07d5c9d0248bf76f72531f57)