x86jit: Cache fpcond in a register.

Mostly to match armjit.
x86jit: Cache the vfpu compare flags in a reg.

Again, to match armjit.
x86jit: Map HI/LO as registers.

Not actually ever cached, but now it's all consistent.
jit: Initialize startDefaultPrefix when switching.
armjit: Fix floor/ceil/cvt.w.s rounding.

Unfortunately, correctly rounding is probably slower.
vertexjit: Specify round to zero in ARM rounding.

Only affects color morphs.  NEON always rounds to zero so let's be
consistent.