lto: use diagnostics_context in print_lto_docs_link

lto: use diagnostics_context in print_lto_docs_link

Properly parse OPT_fdiagnostics_urls_ and then initialize both urls
and colors for global_dc. Doing that we would follow the configure
option --with-documentation-root-url, -fdiagnostics-urls is respected.
Plus we'll print colored warning and note messages.

	PR lto/105364

gcc/ChangeLog:

	* lto-wrapper.cc (print_lto_docs_link): Use global_dc.
	(run_gcc): Parse OPT_fdiagnostics_urls_.
	(main): Initialize global_dc.