/usr/lib/gcc-cross/mipsel-linux-gnu/7/64/libgcc_s.so.1 is in lib64gcc-7-dev-mipsel-cross 7.3.0-16ubuntu3cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gcc-cross/mipsel-linux-gnu/7/64/../../../../../mipsel-linux-gnu/lib64/libgcc_s.so.1