clock_systimer: Replace critical section APIs with spinlock APIs. (64bit only)

If SMP=n or SMP=y && SPINLOCK_IRQ=n, this works in the same way as before.
If SMP=y && SPINLOCK_IRQ=y, performance will be improved.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

sched/clock/clock_systimer.c

@@ -120,9 +120,9 @@ systime_t clock_systimer(void)
 
   /* 64-bit accesses are not atomic on most architectures. */
 
-  flags  = enter_critical_section();
+  flags  = spin_lock_irqsave();
   sample = g_system_timer;
-  leave_critical_section(flags);
+  spin_unlock_irqrestore(flags);
   return sample;
 
 # else /* CONFIG_SYSTEM_TIME64 */