Userland xnr jit

Apple added XO (VM_PROT_EXECUTE_ONLY) in iOS 10 and uses it in JavaScriptCore for JITing.

https://opensource.apple.com/source/JavaScriptCore/JavaScrip...

This doesn't seem any more practical a technique than printing the pages to paper and OCRing them on each access.. one signal and two expensive syscalls per randomly accessed page of memory is hardly cheap, especially in the context of a JIT