JIT memcheck: don't flush register state unless an exception occurs
JIT: support paired load/store with MMU on

Also change the calling convention, to avoid RSCRATCH being clobbered by
memcheck'd loads.
JIT: add "far code emitter" to reduce L1I cache pressure
JIT: use far code emitter in more places
JIT: make memcheck macro support an inverted mode
JIT: fix memcheck with stXx
JIT: fix memcheck on float load/store, plus some corner cases
Merge pull request #927 from FioraAeterna/fastermmu

Fiora's Faster MMU Project