Bug 294513 - hw.ncpu loader(8) tunable for x86 architectures

[jim.chen.1827_gmail.com]

Summary

Most architectures, excluding i386 and amd64, allow you to set artificial limits on the number of logical processors available to FreeBSD through the hw.ncpu variable in loader(8).

This patch increases consistency with other architectures when handling mp_ncpus and mp_maxid through two changes:

1. Allowing cpu_mp_setmaxid() to read the loader(8) tunable and set mp_ncpus and mp_maxid accordingly
2. Preventing mp_start() from changing mp_ncpus through the assign_cpu_ids() method after mp_setmaxid() has already done so