Update for the fact that the first members of the IPI args structures and
pc_cpumask were changed to cpuset_t. This now calculates the cpumask based
on pc_cpuid itself as pc_cpumask is slated for being deorbited. Note that
this needs r221750 to be MFC'ed in order to compile.
This seems to work fine but after a few dozens of successful IPIs something
suddenly adds pc_cpuid to pc_other_cpus, causing the respective assertions
in mp_machdep.c to be triggered when the latter is used as the base for the
targets.