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.
Looking into ``initUniqueStubs`` I'm not exactly sure why it works :/ This may need some more investigation. It definitely doesn't work without the unprotect call though, whether it's before or after.

As for the location of this, it's been a while ago (this was during the SF trip) but afaik we just wanted to initialize it as writable after initializing the MCG (@ptarjan and/or @jdelong may have better memory).