/etc/ld.so.conf.d/x86_64-linux-gnu.conf is in libc6 2.15-0ubuntu10.18.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # Multiarch support
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu
|