Add correct copyright notice wrt DCO

[dkm]

When changes are merged with FSF copyright assignment, the "regular" copyright :

Copyright (C) 20XX-20XX Free Software Foundation, Inc.

is ok.

When using the DCO, we should probably add the same notice as libstc++ is using (and other parts also):

Copyright The GNU Toolchain Authors.

[CohenArthur]

hm, so should we make those changes to all of our files within gcc/rust/ and libgrust/? considering they've now probably all been touched by people using DCO sign-off instead of FSF copyright assignment