JIT+Emitter: support locking flags

This helps us avoid accidentally clobbering flags between two instructions
when the flags are expected to be maintained. Dolphin will of course crash
immediately, but at least it will crash loudly and alert us of the mistake,
instead of forcing hours of bisecting to find the subtle way in which the JIT
has managed to sneak a flag-modifying instruction where there shouldn't be one.
Merge pull request #1166 from FioraAeterna/flaglocking

JIT+Emitter: support locking flags