runtime: refactor obtaining the system stack

The system stack is only needed when we're not on it. So we can directly
call task.SystemStack() without problems.

This also saves a tiny bit of binary size.

src/internal/task/task_none.go

@@ -36,3 +36,9 @@ func OnSystemStack() bool {
 	// This scheduler does not do any stack switching.
 	return true
 }
+
+func SystemStack() uintptr {
+	// System stack is the current stack, so this shouldn't be called.
+	runtimePanic("scheduler is disabled")
+	return 0 // unreachable
+}