Work on MIPS JIT.

Jit still doesn't work (crashes in enter code). Need work on MipsAsm.
Well, I think we should potentially map s0-s7 as fixed registers (either ctx or always v0 or w/e.)

Then, use the rest for reg cache.  But yeah, everything will be mappable from t*/a*, s*, v*, I guess.  Maybe even allow v0/v1 as temps along with at?  That should give like 20 regs or so.  Should be more than enough.

-[Unknown]