[S390] switch sched_clock to store-clock-extended.

Add get_clock_xt to read an 8 byte clock value using store clock
extended (STCKE) and use get_clock_xt for sched_clock. STCKE should
be faster than STCK on newer machines.

Signed-off-by: Jan Glauber <jan.glauber@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index cb232c1..925f9dc 100644
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -66,7 +66,7 @@
  */
 unsigned long long sched_clock(void)
 {
-	return ((get_clock() - jiffies_timer_cc) * 125) >> 9;
+	return ((get_clock_xt() - jiffies_timer_cc) * 125) >> 9;
 }
 
 /*