Threads: Use a dummy idle thread when no other are ready.

This thread will not actually execute instructions, it will only advance the timing/events and try to yield immediately to the next ready thread, if there aren't any ready threads then it will be rescheduled and start its job again.

src/core/hle/kernel/kernel.cpp

@@ -124,6 +124,8 @@ bool LoadExec(u32 entry_point) {
 
     // 0x30 is the typical main thread priority I've seen used so far
     g_main_thread = Kernel::SetupMainThread(0x30);
+    // Setup the idle thread
+    Kernel::SetupIdleThread();
 
     return true;
 }