mips: move support for temporary mappings above KSEG0 to per-CPU data

This is derived from similar work done in r310481 for i386 and r312610 for
armv6/armv7. Additionally, use a critical section to keep the thread
pinned for per-CPU operations instead of completely disabling local interrupts.

No objections from: adrian, jmallett, imp
Differential Revision: https://reviews.freebsd.org/D18593