[PPC] Implement `vsubcuw`

Other half of #2125. I don't know of any title that utilizes this instruction, but I went ahead and implemented it for completeness.

Verified the implementation with `instr__gen_vsubcuw` from #1348. Can be grabbed with:
```
git checkout origin/gen_tests -- src\xenia\cpu\ppc\testing\*vsubcuw.s
```