[Ada] Fix problematic import of type-generic GCC atomic builtin

[Ada] Fix problematic import of type-generic GCC atomic builtin

gcc/ada/

	* gcc-interface/gigi.h (resolve_atomic_size): Declare.
	(list_third): New inline function.
	* gcc-interface/decl.c (type_for_atomic_builtin_p): New function.
	(resolve_atomic_builtin): Likewise.
	(gnat_to_gnu_subprog_type): Perform type resolution for most of
	type-generic GCC atomic builtins and give an error for the rest.
	* gcc-interface/utils2.c (resolve_atomic_size): Make public.