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`

include/asm/util.h

@@ -18,4 +18,4 @@ char const *printChar(int c);
  */
 size_t readUTF8Char(uint8_t *dest, char const *src);
 
-#endif /* RGBDS_UTIL_H */
+#endif // RGBDS_UTIL_H