On iossimulator-arm64 and tvossimulator-arm64, use the
pthread_jit_write_protect_np function to toggle the W^X bit for the current
thread.

Unfortunately, the above function is marked as __API_UNAVAILABLE on ios, tvos,
watchos in the apple headers.  But in the simulator frameworks it is still
present and usable.  So we declare our own prototype of the function and avoid
the clang error.

---

This reverts commit 89a816f, but for IOS
simulator, not for Catalyst.

That commit in turn reverted a9f1207

(ie this current commit is the same as a9f1207 but for ios simulator instead of catalyst)

For Mono, set HOST_DARWIN_SIMULATOR in cmake and C

apparently it wants to see literally `if (__builtin_available(...))`