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/runtime/gc_stack_raw.go

@@ -20,7 +20,7 @@ func markStack() {
 
 	if !task.OnSystemStack() {
 		// Mark system stack.
-		markRoots(getSystemStackPointer(), stackTop)
+		markRoots(task.SystemStack(), stackTop)
 	}
 }