stop using libffi snapshots

For a long time ghc has been shipping snapshots of libffi - originally due to lack of upstream releases it seems.

But there have been quite a few releases now - several lately in the 3.4.x series while ghc still keeps to a 3.3 snapshot. I assume nearly all Linux distros are using official libffi releases already for ghc.

I think it would be better to use an official 3.4.x release tarball instead of some older git snapshot.