gccrs: Add utility function to build proc macro types

gccrs: Add utility function to build proc macro types

Add some utility function to build proc macro entrypoint related types.
Those functions will help generate all required metadata in order for
proc macros to be expanded properly.

gcc/rust/ChangeLog:

	* backend/rust-compile.cc (build_attribute_array): Add a function to
	build the attribute array type.
	(build_derive_proc_macro): Add a function to build the derive proc
	macro type.
	(build_bang_proc_macro): Add a function to build the bang proc macro
	type.
	(build_attribute_proc_macro): Add a function to build the attribute
	proc macro type.
	(build_proc_macro): Add a function to build the proc macro tagged union
	type.
	(build_proc_macro_buffer): Add a function to build the proc macro
	buffer type.
	(build_entrypoint): Add a function to build the proc macro entrypoint
	type.
	* backend/rust-compile.h: Add function prototype.

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