Update to GCC copyright assignment policy

The GCC Steering Committee has decided to
relax the requirement to assign copyright to the Free Software
Foundation. 'Contributors who have an FSF Copyright Assignment don't
need to change anything. Contributors who wish to utilize the Developer
Certificate of Origin should add a Signed-off-by message to their commit
messages. Developers with commit access may add their name to the DCO list
in the MAINTAINERS file to certify the DCO for all future commits in lieu
of individual Signed-off-by messages for each commit.'