use pause in squid.

Reference: /n/atom/patch/applied2013/fsamd64pause2
Date: Tue Sep 17 15:03:25 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/fs/amd64/lock.c	Tue Sep 17 15:02:59 2013
+++ /sys/src/fs/amd64/lock.c	Tue Sep 17 15:03:00 2013
@@ -118,7 +118,7 @@
 	for(;;){
 		splx(x);
 		while(l->sbsem)
-			;
+			pause();
 		x = splhi();
 		if(tas(l) == 0)
 			goto acquire;