pause when halting to avoid using too much power if stuck.

Reference: /n/atom/patch/applied2013/haltpause
Date: Tue Sep 17 01:13:17 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/devarch.c	Tue Sep 17 01:12:54 2013
+++ /sys/src/nix/k10/devarch.c	Tue Sep 17 01:12:54 2013
@@ -402,7 +402,7 @@
 	outb(0xcf9, i|0x06);				/* RST_CPU transition */
 
 	for(;;)
-		;
+		pause();
 }
 
 /*