fixing libffi on NetBSD

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

py-acme (a Let's Encrypt client) is not working on NetBSD-current.
I've fixed a bug in our py-cryptography package, but now we have a
similar problem in py-OpenSSL, and both come via py-cffi from libffi
(at least that's how it looks right now).

libffi's test suite is in bad shape. Over 500 of ~1500 tests fail.

I've made a package for the latest GIT head in wip/libffi-git and
opened an upstream bug report about this, see
https://github.com/libffi/libffi/issues/294

However, if someone here is motivated I expect we can fix it faster
than upstream (who doesn't seem to be very active right now).

Any takers, please? :)
 Thomas