core/sched: tiny logic optimization in idle case

In the case that the no_thread_idle feature is active, the
runqueue_bitcache is checked twice in the case no thread is available to
schedule. This changes the inner while loop to a do-while loop to save
one check from the initial loop iteration, saving a cycle or so in the
idle case.

core/sched.c

@@ -109,9 +109,9 @@ int __attribute__((used)) sched_run(void)
                 active_thread = NULL;
             }
 
-            while (!runqueue_bitcache) {
+            do {
                 sched_arch_idle();
-            }
+            } while (!runqueue_bitcache);
         }
     }