lkl: relax cpu during __ndelay()

__ndelay() spins until the timeout occurs. If interrupts come
concurrently from another host thread, an irq_status bit will be set,
but the handler will not be called during the spin. This behavior is
unreasonable. At least, it differs from that of real Linux running on a
bare-mental machine.

A driver may expect the device to raise an IRQ within a few
microseconds, so it tries to spin and wait for it. This does not work
without the commit.

Calling cpu_relax() explicitly can cause the handlers to fire. This is
also a valid use of cpu_relax() because the CPU is spinning.

Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>

Author: lrh2000

arch/lkl/kernel/time.c

@@ -14,7 +14,7 @@ void __ndelay(unsigned long nsecs)
 	unsigned long long start = lkl_ops->time();
 
 	while (lkl_ops->time() < start + nsecs)
-		;
+		cpu_relax();
 }
 
 void __udelay(unsigned long usecs)