/usr/aarch64-linux-gnu/lib/libm.so.6 is in libc6-arm64-cross 2.19-0ubuntu2cross0.10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/aarch64-linux-gnu/lib/libm-2.19.so
/usr/aarch64-linux-gnu/lib/libm-2.19.so