SMP: Use nproc(1) instead of sysctl kern.smp.cpus

The former respects cpuset, while the latter does not.

$ cpuset -l 1 -- make -V MAKE_JOBS_NUMBER
1

As nproc is newly introduced, keep the previous variant as a fallback
for the time being.

Differential Revision: https://reviews.freebsd.org/D38457