PowerPC/MMU: Refactor to class, move to System.

[AdmiralCurtiss]

This one ended up being pretty large, but it's mostly straightforward too. A few notes of stuff that might look odd:

• The Host functions ended up as static methods in the MMU instead of freestanding ones because they need to access private MMU stuff after getting the MMU instance from the passed guard via System.
• There were a few functions that that aren't actually used anywhere (Read_F64, Read_F32, Write_F64). I've removed those, they were just bitcasts around the unsigned integer variants anyway, so if anyone needs it they can just do that on the caller side.
• The TranslateBatAddress (actually TranslateBatAddess but I fixed the typo) function was previously defined inline in MMU.h but only used inside MMU.cpp, so I've moved it in there.
• I ended up with two different sets of functions for the JITs to call out to. This is a bit silly and definitely refactorable in the future, but it ended up being easier to do this way because the Jit64 uses the ABI_CallFunctionXXX helper functions that always have pointers as their first argument, while the JitArm64 function that calls the MMU uses a very peculiar method of register allocation that seemed prudent not to touch.

[AdmiralCurtiss]

Oh and as usual the ARM changes are untested, so I would appreciate a basic check if stuff still runs.

[JosJuice]

Reviewed and tested.