JIT: use fastmem loads in MMU mode.

Even in games that require MMU mode, loads outside the area specified by
the BAT are rare, so fastmem is a substantial improvement.

All of the interesting changes are in the backpatch handler, to make it
generate DSI exceptions correctly.
JIT: simplify code using MOVTwo.
JIT: use fastmem for stores in MMU mode.

Apparently just works as-is.