Add CVE-2018-20657/binutils

Marking the issue as ignored for stretch.

Marked the issue in accordance with CVE-2018-12698 (as the related
issue) as well already as ignored for jessie assuming the LTS team is
fine with that move (If not I can revert the change).

Add reference to upstream issue, which likely will only be adressed in
the bullseye release cycle anyway and then merged into binutils.