x86jit: Free up a reg and cut some bytes from asm

This is an even easier way to free up a reg: normally we don't need a full 64-bit offset for the jit base.

Of course, in 32-bit, we can't free up a reg for CTXREG anyway, since there's no MEMBASEREG there.

That adds a byte to the main asm loop, but cutting the constants takes 6, so it's still a 5 byte savings.  Probably makes no real difference, though.

-[Unknown]