Initial MIPS emitter

This only has a few basic functions right now.  Intended to support MIPS32 and MIPS64.

I had some comments on the initial jit but it's not ready yet anyway: https://github.com/unknownbrackets/ppsspp/pull/4

-[Unknown]
@Sonicadvance1 would you have an MMIXXEmitter?

-[Unknown]