target-mips: fix for incorrect multiplication with MULQ_S.PH

The change corrects sign-related issue with MULQ_S.PH. It also includes
extension to the already existing test which will trigger the issue.

Signed-off-by: Petar Jovanovic <petarj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
target-mips: fix for sign-issue in MULQ_W helper

Correct sign-propagation before multiplication in MULQ_W helper.
The change also fixes previously incorrect expected values in the
tests for MULQ_RS.W and MULQ_S.W.

Signed-off-by: Petar Jovanovic <petarj@mips.com>
Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>