go: update builtin function attributes

	PR go/118746
	* go-gcc.cc (class Gcc_backend): Define builtin_cold,
	builtin_leaf, builtin_nonnull.  Alphabetize constants.
	(Gcc_backend::Gcc_backend): Update attributes for builtin
	functions to match builtins.def.
	(Gcc_backend::define_builtin): Split out attribute setting into
	set_attribtues.
	(Gcc_backend::set_attribtues): New method split out of
	define_builtin.  Support new flag values.