complibs: hide companion libraries for target entry

The companion libraries on the target are required only for internal use by
binutils and gdb. The user should not have to know about this, so hide the
option.

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}