JIT: unify JitAsm for JIT64 and JIT64IL.

Having a bunch of nearly identical code sitting around isn't a good idea,
so fix that. JITIL had a couple of extra entry points into the JIT main
loop, so adjust the code appropriately.

Also fixes Issue 7186.
Merge pull request #309 from magumagu/jit-unify-asm

JIT: unify JitAsm for JIT64 and JIT64IL.