/usr/src/gcc-4.9/debian/shlibs.commonx32 is in gcc-4.9-source 4.9.3-13ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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libgcc_s 1 libx32gcc1 libstdc++ 6 libx32stdc++6