Broken on arm64

[martijnthe]

See https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/Major64-BitChanges/Major64-BitChanges.html

Under "Impact of Function Call Changes on Your App":

"When you cast a function, you must be careful that the cast version of the function has the same signature as the original function. In particular, avoid casting a a function to a form that takes a different set of parameters (such as casting a function pointer that takes a variadic pattern to one that takes a fixed number of parameters)."

I think it's probably possible to fix it up, since it's possible to get the number and types of arguments, but it's not going to be pretty. Probably it's better to wait for libffi to catch up with iOS AArch64 and use that instead.