Update to GCC copyright assignment policy

The GCC Steering Committee has decided torelax the requirement to assign copyright to the Free SoftwareFoundation. "Contributors who have an FSF Copyright Assignment don'tneed to change anything. Contributors who wish to utilize the DeveloperCertificate of Origin should add a Signed-off-by message to their commitmessages. Developers with commit access may add their name to the DCO listin the MAINTAINERS file to certify the DCO for all future commits in lieuof individual Signed-off-by messages for each commit."