makechrootpkg: parse default_makepkg_args as well

We're looping through ${@:OPTIND} rather than ${makepkg_args[@]} for parsing currently, thus excluding options in ${default_makepkg_args[@]}. However, ${default_makepkg_args[@]} may contain options that need parsing (and it does at the moment). So, looping through ${makepkg_args[@]} could be a better choice.