target-mips: fix multiplication in mipsdsp_rndq15_mul_q15_q15

target-mips: fix multiplication in mipsdsp_rndq15_mul_q15_q15

Multiplication of Q15 fractional halfword vectors was incorrect in the
previous implementation of mipsdsp_rndq15_mul_q15_q15. It failed to take
element signs into account. This change fixes it, and it adds a test case
for it.

The change also removes unnecessary cast in the function
mipsdsp_mul_q15_q15_overflowflag21().

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Reviewed-by: Richard Henderson  <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>