can do objprimspec at spesh-time
but please without debug spam.
Add MVM_USED_BY_JIT and add it in one place.

This disables optimizations on a per-function basis that fiddle with
calling conventions and so frustrate JIT compilation's calls to them.
Only annotated the one that made things explosive on Windows so far,
but really we should mark up all that the JIT uses.
Do more checking before speshing arg unboxing.

We could end up trying to unbox a num as an int before. While we can
spesh that situation, for now just make sure we apply the transform
when it really is safe.
Merge branch 'master' into moar-jit
Merge branch 'moar-jit' into HEAD