armemu: Join QADD16/QSUB16 and fix saturation clamping.

This merges the instructions, since they do the same thing, just one adds while the other subtracts.

This also fixes the lower-bounds clamping. Below shows what the hardware and emulator used to report.

- What the hardware reported
![hw](http://i.imgur.com/z9IIeOj.jpg)

- What the emulator reported:
![emu](http://i.imgur.com/A9in8ZR.png)
`std::min(lo_result, -0x8000);`?