[kernel] Use PAUSE in spinwait

Using PAUSE in a tight loop allows other logical cores on the same
physical core to make use of more of the core's resources.

src/kernel/clock.cpp

@@ -17,6 +17,6 @@ void
 clock::spinwait(uint64_t us) const
 {
 	uint64_t when = value() + us;
-	while (value() < when);
+	while (value() < when) asm ("pause");
 }