libffi-3.4.7 bug

Hi!

libffi-3.4.7 unfortunately had a bug snuck in that I'd reported a few
days prior to the release. See https://github.com/libffi/libffi/issues/879.

It causes test failures with Clang at least. Reverting it is non-trivial
because a few build system and test tweaks were made, but you can use
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/libffi/files/libffi-3.4.7-Revert-Fix-x86-ffi64-calls-with-6-gp-and-some-sse-re.patch?id=913b9ae63372f13deb88137f646213574ae94158
if you like.

thanks,
sam