Implement a few more x86jit ops

I'm not 100% sure about d7bdded6f89c71e1f7b414e68f860edb5298ce63.  If there's a special reason it wasn't using `extrabytes` it probably needs a comment.

I initially added ROUNDSD etc. but then realized of course I need vf2i to result in an *integer*, heh.  So I never used them.  I think they work but I only really tested ROUNDSD and not heavily at all.

-[Unknown]