target-mips: implement DSP (d)append sub-class with TCG

DSP instruction from the (d)append sub-class can be implemented with
TCG. Use a different function for these instructions are they are quite
different from compare-pick sub-class.

Fix BALIGN instruction for negative value, where the value should be
zero-extended before being shift to the right.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>