Tweak update-copyright.py script

Tweak update-copyright.py script

When running update-copyright.py --this-year, I've encountered various
failures, this patch works around those.

2026-01-02  Jakub Jelinek  <jakub@redhat.com>

	* update-copyright.py (LibPhobosFilter): Ignore also
	__importc_builtins.di.
	(LibGRustFilter): New filter.
	(GCCCopyright): Add several external authors.
	(GCCCmdLine): Use LibGRustFilter for libgrust.  Add libga68
	directory, both to list of directories and default directories.