[S390] get_clock inline assembly.

Add missing volatile to the get_clock / get_cycles inline assemblies
to avoid that consecutive calls get optimized away.

Signed-off-by: Andreas Krebbel <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

include/asm-s390/timex.h

···
 {
 	cycles_t cycles;
 
-	__asm__("stck 0(%1)" : "=m" (cycles) : "a" (&cycles) : "cc");
+	__asm__ __volatile__ ("stck 0(%1)" : "=m" (cycles) : "a" (&cycles) : "cc");
 	return cycles >> 2;
 }
 
···
 {
 	unsigned long long clk;
 
-	__asm__("stck 0(%1)" : "=m" (clk) : "a" (&clk) : "cc");
+	__asm__ __volatile__ ("stck 0(%1)" : "=m" (clk) : "a" (&clk) : "cc");
 	return clk;
 }