As summary says, it's at keywordreq state in bug 272046. Depending on sys-devel/gcc[libffi] isn't a solution, because that gets broken when user does a gcc switch using gcc-config. Or if it's using an internal copy, that's definately wrong.
virtual/libffi is keyworded, there is nothing blocking you from using it. please stop shipping an internal copy.
this doesn't seem sane after all, given that libffi just changed ABI too... i'm going to just close this