This CL adds support of call injection and async preemption on
MIPS and MIPS64.

Like ARM64, we need to clobber one register (REGTMP) for
returning from the injected call. Previous CLs have marked code
sequences that use REGTMP async-nonpreemtible.

It seems on MIPS/MIPS64, a CALL instruction is not "atomic" (!).
If a signal is delivered right at the CALL instruction, we may
see an updated LR with a not-yet-updated PC. In some cases this
may lead to failed stack unwinding. Don't preempt in this case.

Change-Id: I99437b2d05869ded5c0c8cb55265dbfc933aedab
Reviewed-on: https://go-review.googlesource.com/c/go/+/203720Reviewed-by: Keith Randall <khr@golang.org>