Make comments more consistent

- Changes most `/* comments */` to `// comments`
- Changes `/**` block comments consistently to `/*`
- Adds consistent license comments to all files

Also renames `T_POP_SET` to `T_Z80_SET`

src/linkdefs.c

@@ -1,3 +1,10 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
 
 #include "linkdefs.h"