Micro-optimize sleepq_broadcast().

• Avoid a conditional branch on the return value of sleepq_resume_thread() by ORing its return value into the boolean wakeup_swapper. This is consistent with other sleepqueue functions which just pass this return value to their caller.
• sleepq_resume_thread() unconditionally removes the thread from its queue, so there's no need to maintain a pointer to the next element in the queue.

MFC after: 2 weeks