DSA-3889 libffi - security update

libffi, a library used to call code written in one language from code written
in a different language, was enforcing an executable stack on the i386
architecture. While this might not be considered a vulnerability by itself,
this could be leveraged when exploiting other vulnerabilities, like for example
the stack clash class of vulnerabilities discovered by Qualys Research Labs.
For the full details, please refer to their advisory published at:
https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt