(build_objc_string_object): Build a full declaration if not using the next
runtime.
(objc_add_static_instance): New function.
(init_module_descriptor): Add reference to static instances table.
(build_module_descriptor): Add field for static instances table.
(get_objc_string_decl): New function.
(generate_static_references): New function.
(finish_objc): Call generate_static_references if needed.