rt: Only wake up all schedulers when no tasks are left

At the moment there's not really any reason to be raising this signal,
since they schedulers wake up periodically anyway, but once we remove
the timer this will be how the schedulers know to exit.

src/rt/rust_kernel.cpp

@@ -189,11 +189,9 @@ rust_kernel::release_task_id(rust_task_id id) {
     task_table.remove(id);
 }
 
-void rust_kernel::wakeup_schedulers() {
+void rust_kernel::exit_schedulers() {
     for(size_t i = 0; i < num_threads; ++i) {
-        rust_scheduler *sched = threads[i];
-        scoped_lock with(sched->lock);
-        sched->lock.signal_all();
+        threads[i]->exit();
     }
 }