On Apple arm64, theABI for a variardic function is different than the
ABI for a fixed-arg function, even of the same arg types.

In order to account for this, CFFI must call ffi_prep_cif_var for
variardic calls.

see: https://developer.apple.com/library/archive/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html

Use the system libffi if deployment target is at least 10.15