Kernel: Stop idling after handling an IRQ

If we receive an IRQ while the idle task is running, prevent it from
re-halting the CPU after the IRQ handler returns.

Instead have the idle task yield to the scheduler, so we can see if
the IRQ has unblocked something.

Kernel/init.cpp

@@ -277,8 +277,6 @@ extern "C" [[noreturn]] void init()
 
     sti();
 
-    // This now becomes the idle process :^)
-    for (;;) {
-        asm("hlt");
-    }
+    Scheduler::idle_loop();
+    ASSERT_NOT_REACHED();
 }