LinuxKPI: implement cpumask_of()

Add a static set of cpumasks for all (possible) cpus with only the one
indexed cpu enabled in each set.
This is needed for cpumask_of(_cpuid) which returns a cpumask (cpuset)
with only cpu _cpuid enabled and is used by one wireless driver at least.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37223

(cherry picked from commit e2361e04b02aafa927c32c4f5a39e6fd96196082)