arch_alarm: don't init local variable for fpu test case fail

D16 register will be cleared when compiling with gcc on some boards.

Signed-off-by: zhangyuan21 <zhangyuan21@xiaomi.com>

drivers/timers/arch_alarm.c

@@ -319,7 +319,7 @@ int weak_function up_alarm_tick_start(clock_t ticks)
 
   if (g_oneshot_lower != NULL)
     {
-      clock_t now = 0;
+      clock_t now;
       clock_t delta;
 
       ONESHOT_TICK_CURRENT(g_oneshot_lower, &now);