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, something like

```cpp
#ifndef __APPLE__
if (pthread_attr_getstack(...) != 0) {
  always_assert(false);
}
#else
apple_get_stack_limits(...);
#endif
```