Refactor FourCC magic uses

- Use new fourcc_t type
- Improves compiler compatibility by removing multi chars

src/xenia/kernel/xam/apps/xam_app.cc

@@ -52,7 +52,7 @@ X_HRESULT XamApp::DispatchMessageSync(uint32_t message, uint32_t buffer_ptr,
       if (!e || !buffer || !extra) {
         return X_E_INVALIDARG;
       }
-      assert_true(extra->magic == 'XEN\0');
+      assert_true(extra->magic == kXObjSignature);
       if (data->buffer_size) {
         std::memset(buffer, 0, data->buffer_size);
       }

src/xenia/kernel/xam/xam_content_aggregate.cc

@@ -95,7 +95,7 @@ dword_result_t XamContentAggregateCreateEnumerator(qword_t xuid,
     return result;
   }
 
-  extra->magic = 'XEN\0';
+  extra->magic = kXObjSignature;
   extra->handle = e->handle();
 
   if (!device_info || device_info->device_type == DeviceType::HDD) {