Expose threads-per-core and physical core count information

With new sysctls (to the best of our ability do detect them). Restructured
smp.4 slightly for clarity (keep relevant stuff closer to the top) while
documenting.

Reviewed by: markj, jhibbits (ppc parts)
MFC after: 3 days
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D18322