scripts/populate: fix handling the forced libraries list-file

If a list-file is used, then each library in the file will be handled
twice (not a real issue, as the second iteration will find the library
already present, just avoid doing the job twice).
(transplanted from 7fec5db8933cb296c665b8526f27a779aa0c0c7a)

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

    if [ "${CT_ARCH_64}" = "y" ]; then

        CT_TARGET_ARCH="s390x"

    fi

}