[All] Fixed multiple issues during build on Linux

- Added some fixes introduced by RodoMa92 in PR198

- Lack of AVX2 extension (should be done differently in the future)
- Disable deprecated-volatile warning
- Added missing override in posix EventInfo, ImGui notification class and XContent class
- Removed not used XAudio2.h include in XMP
- Fixed missing switch-case in XObject
- Added fugly template in native_list.h
- Fixed multiple smaller issues

src/xenia/kernel/xobject.h

@@ -135,6 +135,8 @@ class XObject {
       case Type::Thread:
       case Type::Timer:
         return true;
+      default:
+        return false;
     }
     return false;
   }