Refactor FourCC magic uses

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

src/xenia/apu/audio_system.h

@@ -23,6 +23,8 @@
 namespace xe {
 namespace apu {
 
+constexpr fourcc_t kAudioSaveSignature = make_fourcc("XAUD");
+
 class AudioDriver;
 class XmaDecoder;