Merged in masayuki2009/nuttx.nuttx/clock_systimer (pull request #570)

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

Approved-by: Gregory Nutt <gnutt@nuttx.org>

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 */