gccrs: Emit error with proc macro on non functions

gccrs: Emit error with proc macro on non functions

An error should be emitted when a proc_macro, proc_macro_attribute or
proc_macro_derive attribute is met on any non function item. Those
attribute shall be placed only on bare functions.

gcc/rust/ChangeLog:

	* util/rust-attributes.cc (is_proc_macro_type): Add function to
	identify matching proc macro attribute type.
	(check_proc_macro_non_function): Add function to check and emit
	error.
	(AttributeChecker::visit): Add cal to new check function.

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