Change the x86emitter jmp emitter from using s32 to sptr.

This doesn't change anything on x86_32, but it is required on x86_64 so we don't lose precision.
Also adds an assert to see if the distance is greater than the maximum 32bit jump, which can't be hit on x86_32, but can be on x86_64.
Merge pull request #135 from Sonicadvance1/jmp_emitter

Change the x86emitter jmp emitter from using s32 to sptr.