Refactor FourCC magic uses

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

src/xenia/emulator.h

@@ -44,6 +44,8 @@ class Window;
 
 namespace xe {
 
+constexpr fourcc_t kEmulatorSaveSignature = make_fourcc("XSAV");
+
 // The main type that runs the whole emulator.
 // This is responsible for initializing and managing all the various subsystems.
 class Emulator {