at a user mode linux image I observed it. Reproducible: Always
Created attachment 293623 [details] build log
old releases of libffi are never intresting unless you have a patch. try 3.0.11_rc2 (or preferably upstream git) the discussion about libffi-3.0.11_rc2 and git test failures is going on at "libffi-discuss" mailing list: http://sourceware.org/libffi/ so they can be fixed in time for 3.0.11 release.
dev-libs/libffi-3.0.11_rc2 passes tests fine