/usr/src/gcc-6/debian/rules.d/ is in gcc-6-source 6.4.0-17ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/src/gcc-6/debian/rules.d/binary-ada.mk
/usr/src/gcc-6/debian/rules.d/binary-base.mk
/usr/src/gcc-6/debian/rules.d/binary-cpp.mk
/usr/src/gcc-6/debian/rules.d/binary-cxx.mk
/usr/src/gcc-6/debian/rules.d/binary-d.mk
/usr/src/gcc-6/debian/rules.d/binary-fixincl.mk
/usr/src/gcc-6/debian/rules.d/binary-fortran.mk
/usr/src/gcc-6/debian/rules.d/binary-gcc.mk
/usr/src/gcc-6/debian/rules.d/binary-go.mk
/usr/src/gcc-6/debian/rules.d/binary-hppa64.mk
/usr/src/gcc-6/debian/rules.d/binary-java.mk
/usr/src/gcc-6/debian/rules.d/binary-libasan.mk
/usr/src/gcc-6/debian/rules.d/binary-libatomic.mk
/usr/src/gcc-6/debian/rules.d/binary-libcc1.mk
/usr/src/gcc-6/debian/rules.d/binary-libcilkrts.mk
/usr/src/gcc-6/debian/rules.d/binary-libgcc.mk
/usr/src/gcc-6/debian/rules.d/binary-libgccjit.mk
/usr/src/gcc-6/debian/rules.d/binary-libgomp.mk
/usr/src/gcc-6/debian/rules.d/binary-libitm.mk
/usr/src/gcc-6/debian/rules.d/binary-liblsan.mk
/usr/src/gcc-6/debian/rules.d/binary-libmpx.mk
/usr/src/gcc-6/debian/rules.d/binary-libobjc.mk
/usr/src/gcc-6/debian/rules.d/binary-libquadmath.mk
/usr/src/gcc-6/debian/rules.d/binary-libssp.mk
/usr/src/gcc-6/debian/rules.d/binary-libstdcxx.mk
/usr/src/gcc-6/debian/rules.d/binary-libtsan.mk
/usr/src/gcc-6/debian/rules.d/binary-libubsan.mk
/usr/src/gcc-6/debian/rules.d/binary-libvtv.mk
/usr/src/gcc-6/debian/rules.d/binary-neon.mk
/usr/src/gcc-6/debian/rules.d/binary-nof.mk
/usr/src/gcc-6/debian/rules.d/binary-objc.mk
/usr/src/gcc-6/debian/rules.d/binary-objcxx.mk
/usr/src/gcc-6/debian/rules.d/binary-snapshot.mk
/usr/src/gcc-6/debian/rules.d/binary-softfloat.mk
/usr/src/gcc-6/debian/rules.d/binary-source.mk