Optimise load_avg and recent_cpu updates

- only run when BSD scheduler is enabled

src/threads/thread.c

@@ -155,7 +155,7 @@ thread_tick (void)
 
   /* Update system load_avg and all threads recent_cpu every second. */
   int64_t ticks = timer_ticks ();
-  if (ticks % TIMER_FREQ == 0)
+  if (thread_mlfqs && (ticks % TIMER_FREQ == 0))
     {
       fp32_t old_coeff = fp_mul (fp_div_int (int_to_fp (59), 60), load_avg);
       fp32_t new_coeff = fp_div_int (int_to_fp (threads_ready ()), 60);
@@ -165,7 +165,7 @@ thread_tick (void)
     }
 
   /* Update current thread's recent_cpu. */
-  if (t != idle_thread)
+  if (thread_mlfqs && (t != idle_thread))
     {
       t->recent_cpu = fp_add_int (t->recent_cpu, 1);
       if (ticks % 4 == 0) // recent_cpu was updated, update priority.