The old neon-vfpu branch, rebased on master

Still doesn't quite work, so disabled by default. But having it in master will make it easier to work on and less likely to drift way off like the old branch did. 

I tried to do a proper rebase of it keeping the commits but failed due to all the drift, so I split it into a few pieces, some of which were merged previously (like the emitter changes), and the rest is here, manually tweaked to fit in with the current code.
Maybe should be MIPSReg?  Might want the typesafety.  Might even be worth a `void R(ARMReg)` with no body just to prevent it using the wrong one (same probably in x86 jit, have to use `::R()`.)

-[Unknown]