/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/ is in gcc-6-plugin-dev-i686-linux-gnu 6.4.0-17ubuntu1cross1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/att.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/gnu-user-common.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/gnu-user64.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/i386-opts.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/i386-protos.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/i386.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/linux-common.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/linux64.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/stringop.def
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/unix.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/x86-64.h
/usr/lib/gcc-cross/i686-linux-gnu/6/plugin/include/config/i386/x86-tune.def