Warnings: Add UNREACHABLE macros to switches that contemplate all possible values.

src/core/hw/gpu.h

@@ -74,9 +74,9 @@ struct Regs {
         case PixelFormat::RGB5A1:
         case PixelFormat::RGBA4:
             return 2;
-        default:
-            UNIMPLEMENTED();
         }
+
+        UNREACHABLE();
     }
 
     INSERT_PADDING_WORDS(0x4);