Config: Port MMU setting to new config system.

Source/Core/Core/System.h

@@ -29,6 +29,7 @@ public:
   void Initialize();
 
   bool IsDualCoreMode() const { return m_separate_cpu_and_gpu_threads; }
+  bool IsMMUMode() const { return m_mmu_enabled; }
 
 private:
   System();
@@ -37,5 +38,6 @@ private:
   std::unique_ptr<Impl> m_impl;
 
   bool m_separate_cpu_and_gpu_threads = false;
+  bool m_mmu_enabled = false;
 };
 }  // namespace Core