scheduler: halt and wait for irq if nothing was scheduleable
scheduler: halt and wait for irq if nothing was scheduleable

h/t tankenmate@hackernews who did this a bit more verbosely
https://news.ycombinator.com/item?id=6974384

cpehle/xv6,maxymania/open-xv6,cpehle/xv6,maxymania/open-xv6