[IA64] improve __cpu_die()

use udelay instead of nop loop.
This patch reduces __cpu_die() time much.

Signed-off-by: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>

diff -r b4adb5049860 xen/arch/ia64/linux-xen/smpboot.c
--- a/xen/arch/ia64/linux-xen/smpboot.c	Thu Jul 24 14:24:39 2008 +0900
+++ b/xen/arch/ia64/linux-xen/smpboot.c	Thu Jul 24 14:24:54 2008 +0900
@@ -748,9 +748,7 @@
 			return;
 		}
 #ifdef XEN
-		/* XXX: There must be a better way to sleep */
-		for (int j = 0; j < 1000000; j++)
-			cpu_relax();
+		udelay(100 * 1000);
 #else
 		msleep(100);
 #endif