[RFC] PowerPC flags emulation optimization

https://github.com/delroth/dolphin/commit/81404494599f9d783c3a46edc44470c372ff9f90 explains in details what this is about.

Fairly nice performance boost for CPU emulation. @JMC47 reports a 5-12% performance improvement in CPU limited situations.

Caveats: currently broken for JITIL64, JITARM and JITILARM. Since it needs to touch interpreter code, it's difficult to isolate the change from these 3 JITs, and they will have to be modified.
You can just write (u64)(s32)value.