These aren't used to modify the data they point to, so make that
explicit. Also while we're at it, add const to any nearby variables that
can be made so.
PowerPC.h at this point is pretty much a general glob of stuff, and it's
unfortunate, since it means pulling in a lot of unrelated header
dependencies and a bunch of other things that don't need to be seen by
things that just want to read memory.
Breaking this out into its own header keeps all the MMU-related stuff
together and also limits the amount of header dependencies being
included (the primary motivation for this being the former reason).
It only needs to be updated when we changes the symbols, not every time the code widget updates and it does take a while to update them so this fixes some delay when updating the code window.
