JIT: make fused multiply-add rounding more correct

At least going by the softfp branch, it looks like the Gamecube CPU doesn't
keep extra precision between steps, so we need to clamp to approximate its
behavior.

Also fix the interpreter and paired instructions to match, though I don't
know if the latter is necessary for any games.

As far as I can tell, if I'm not totally crazy, this fixes Mario Kart Wii's replays.