Refactor FourCC magic uses

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

src/xenia/gpu/xenos.h

@@ -15,6 +15,7 @@
 #include "xenia/base/assert.h"
 #include "xenia/base/byte_order.h"
 #include "xenia/base/math.h"
+#include "xenia/base/memory.h"
 #include "xenia/base/platform.h"
 
 namespace xe {
@@ -27,6 +28,8 @@ namespace xenos {
 // in bit fields (registers are 32-bit, and the microcode consists of triples of
 // 32-bit words).
 
+constexpr fourcc_t kSwapSignature = make_fourcc("SWAP");
+
 enum class ShaderType : uint32_t {
   kVertex = 0,
   kPixel = 1,