System: Mark the VM as Stopping in ShutdownSystem unconditionally

Fixes an issue where Discord RPC didn't revert to "No Game Running"
on shutdown.

src/core/system.cpp

@@ -4138,9 +4138,8 @@ void System::ShutdownSystem(bool save_resume_state)
   if (save_resume_state)
     SaveResumeState();
 
-  if (s_system_executing)
   s_state = State::Stopping;
-  else
+  if (!s_system_executing)
     DestroySystem();
 }