We now have two darwin flavours. AArch64-Darwin, and
x86_64-darwin, the latter one which has proper custom
adjustor support, the former though relies on libffi.

Mixing both leads to odd crashes, as the closures might
not fit the size of the libffi closures. Hence this
needs to be guarded by the USE_LBFFI_FOR_ADJUSTORS guard.

Original patch by Hamish Mackenzie

(cherry picked from commit a6a8d3f5)