target/openrisc: Streamline arithmetic and OVE

Fix incorrect overflow calculation.  Move overflow exception check
to a helper function, to eliminate inline branches.  Remove some
incorrect special casing of R0.  Implement multiply inline.

Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <rth@twiddle.net>
