Config: Port dual core setting to new config system.

Source/Core/Core/System.h

@@ -26,10 +26,16 @@ public:
     return instance;
   }
 
+  void Initialize();
+
+  bool IsDualCoreMode() const { return m_separate_cpu_and_gpu_threads; }
+
 private:
   System();
 
   struct Impl;
   std::unique_ptr<Impl> m_impl;
+
+  bool m_separate_cpu_and_gpu_threads = false;
 };
 }  // namespace Core