threads: correct 'thrd_yield()'

threads: correct 'thrd_yield()'

The C11 thrd_yield() interface should return void. Also, it doesn't need
to do anything in our model-checker for now. I'm leaving its
implementation commented out for now, in case we find it helps for
fairness, for instance. Note that I made its location parameter non-NULL
now, too, so that we can identify the Thread, if we ever use it.