target-tricore: fix msub32_suov return wrong results

If the signed result of the multiplication overflows, we would get a negative
value, which would result in a addition instead of a subtraction.

Now we do the overflow calculation and saturation by hand instead of using
suov32_neg.

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