Exclude the X64Emitter test on ARM.

Don't want to make that run on ARM...
jit: Make available js_ and jo_ in regcaches.
Fix a sign comparison warning.

This isn't really negative anyway.
jit: Improve and unify GPR spill logic.

Now the same logic on x86 and ARM, and handles HI/LO/etc. better.
Don't allow clobbering in a likely delay slot.

Since, we don't know which path it will take.  Also, don't clobber when
it's the delay slot of the instruction in question - this was a tricky
case.
jit: Discard clobbered registers on spill.

If we're spilling anyway, discard rather than saving.
Merge pull request #7151 from unknownbrackets/jit-minor

jit: Prioritize clobbered regs and discard them on spill