/etc/ld.so.conf.d/x86_64-linux-gnu.conf is in libc6 2.19-18+deb8u10.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Multiarch support /lib/x86_64-linux-gnu /usr/lib/x86_64-linux-gnu