gccrs: Change ABI setup and add gccrs_proc_macro attr

gccrs: Change ABI setup and add gccrs_proc_macro attr

Change the way the ABI is setup on a function to avoid duplicates. ABI
is setup by the setup function only now. Add a new attribute to the
function "gccrs_proc_macro" in order to differentiate it from another
type of function.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (handle_proc_macro_common): Add
	new attribute "gccrs_proc_macro" to all procedural macro
	functions.
	(get_abi): Add a function to retrieve the correct ABI depending
	on wether the function is a proc macro or not.

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