libffi: add aarch64 support from upstream

Fixes build failures like:

  http://autobuild.buildroot.org/results/6b4b4c414d366c936df480747c78272050c84293/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>