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]
The "X" in MIPSXEmitter is pretty redundant, for some reason @Sonicadvance1 copied it from my x86 emitter (where it stands for X86) into the arm emitter becoming "ARMXEmitter", let's not keep that up :P