Delte literal 2 special path
Add comment for gen_exit_patch
16 bit align in jmp target see https://github.com/miura1729/mruby/wiki/%E6%A9%9F%E6%A2%B0%E8%AA%9E%E6%9C%80%E9%81%A9%E5%8C%96%E3%83%A1%E3%83%A2