target-m68k: add 680x0 divu/divs variants

Update helper to set the throwing location in case of div-by-0.
Cleanup divX.w and add quad word variants of divX.l.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twidle.net>
[laurent: modified to clear Z on overflow, as found with risu]
