armemu: Join QADD16 and QSUB16 together.

The only difference between these ops is one adds and one subtracts. Everything is literally the same.
armemu: Fix lower-bound signed saturation clamping for QADD16/QSUB16.
Merge pull request #287 from lioncash/qaddsub16

armemu: Join QADD16/QSUB16 and fix saturation clamping.