Add FreeBSD amd64 support to ABI check

We've carried this patch [1] in all FreeBSD python ports that bundle libffi since ~2006. It's time it was upstreamed. I will submit a separate Python issue for their bundled version pending a future merge of libffi updates.

[1] https://svnweb.freebsd.org/ports/head/lang/python33/files/patch-Modules-_ctypes-libffi-configure?revision=340725&view=markup