libffi: fix v3.3 compile on ppc64le

The latest released version of libffi no longer compiles on ppc64le
based machines. Some searching found a patch that fixed our issue but
had not been submitted upstream to libffi.

It has now been submitted upstream with this PR:
https://github.com/libffi/libffi/pull/561



Signed-off-by: Andrew Geissler <geissonator@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>