Update to GCC copyright assignment policy

[Joseph Myers]

On Tue, 1 Jun 2021, David Edelsohn via Gcc wrote:

> The copyright author will be listed as "Free Software Foundation,
> Inc." and/or "The GNU Toolchain Authors", as appropriate.

And copyright notices naming "The GNU Toolchain Authors" should not 
include a date - that's following the recommendations at 
https://www.linuxfoundation.org/en/blog/copyright-notices-in-open-source-software-projects/ 
for the form of copyright notices in projects with many copyright holders.

-- 
Joseph S. Myers
joseph@codesourcery.com