Go Gcc_backend class: mark prefetch as novops

	PR go/94607
	* go-gcc.cc (class Gcc_backend): Define builtin_const,
	builtin_noreturn, builtin_novops.
	(Gcc_backend::define_builtin): Change const_p and noreturn_p
	parameters to a single flags parameter.  Change all callers.
	(Gcc_backend::Gcc_backend): Pass novops for prefetch.