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.
@danslo Here are some internal comments from @oyamauchi 

> Shouldn't whatever needs to write to the TC call this just before it writes? Why do we need to do this at process init time And wait, why does it work if this is done *after* initUniqueStubs? That writes to the TC.