gccrs: Check proc_macro attributes on non root functions

gccrs: Check proc_macro attributes on non root functions

Check proc_macro, proc_macro_attribute and proc_macro_derive attributes
on non root functions, emit an error when one is found.

gcc/rust/ChangeLog:

	* util/rust-attributes.cc (check_proc_macro_non_root): Add
	function to emit this specific error.
	(AttributeChecker::visit): Modify visitor to propagate to some
	containers that were not handled correctly.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>