/usr/aarch64-linux-gnu/lib/libm.so is in libc6-dev-arm64-cross 2.27-3ubuntu1cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/aarch64-linux-gnu/lib/libm.so.6
/usr/aarch64-linux-gnu/lib/libm.so.6