/usr/src/gcc-5/debian/libquadmath.symbols is in gcc-5-source 5.5.0-12ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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libquadmath.so.0 #PACKAGE# #MINVER# (symver)QUADMATH_1.0 4.6