Config: Port CPU overclock setting to new config.

[AdmiralCurtiss]

Mostly so I can get a review on if this is the correct way to do it to avoid losing perf in hot code, in particular the config change callback.

[JosJuice]

The implementation seems reasonable. But have we tested that there actually is any noticeable performance loss when doing this without the second commit?

[AdmiralCurtiss]

re perf, as always this is very unscientific but idling in the Melee main menu I get roughly:

before: ~390 FPS
without last commit: ~370 FPS
with last commit: ~390 FPS

[JosJuice]

Alright, it definitely sounds like we should go with the caching then, considering it's a fairly simple thing to add.