JIT on OSX

As discussed with @ptarjan, these don't fix the JIT entirely (though you can now do a JITted ``echo 1`` on OSX), but these are fine to land on their own.
Yeah, there are too many unknowns here for my liking. If, as you say, the default permissions for mmap'ed memory are different between OS X and Linux, then I think it makes more sense to explicitly set the permissions to what we expect, immediately after calling mmap().

It may also be the case that you've found a legit bug that's being hidden on Linux and exposed on OS X somehow, in which case I'd like to get to the bottom of it.