Some armjit sync (warning: untested)

I just went over and compared to see if I could see anything obviously wrong / different.  I can't test the armjit though, so probably don't merge without testing.  Mostly it's:

 * Replace `rs == 0` with `gpr.IsImm(rs) && gpr.GetImm(rs) == 0`.
 * Add checks for writes to zero.
 * Remove one redundant line since reg 0 always IsImm.
 * Remove an init to rd in ext (ext always writes the value.)
 * R10 -> CTXREG in more places.
 * Some indentation cleanup.
 * Disable ext since it's reported to break Disgaea (although not sure, we can re-enable if not.)

-[Unknown]