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/warning.h

@@ -91,4 +91,4 @@ _Noreturn void fatalerror(char const *fmt, ...) format_(printf, 1, 2);
  */
 void error(char const *fmt, ...) format_(printf, 1, 2);
 
-#endif
+#endif // WARNING_H