target/mips/mxu: Add Q16SCOP instruction

The instruction is used to determine sign of four 16-bit
packed data in parallel.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
Message-Id: <20230608104222.1520143-31-lis8215@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>