[Emulator] Launch file based on signature instead of file extension

src/xenia/kernel/user_module.h

@@ -31,6 +31,8 @@ class XThread;
 namespace xe {
 namespace kernel {
 
+constexpr fourcc_t kEXESignature = make_fourcc('M', 'Z', 0, 0);
+
 class UserModule : public XModule {
  public:
   UserModule(KernelState* kernel_state);