scripts: fix missing space when using CT_EXTRA_FLAGS_FOR_HOST

CT_EXTRA_FLAGS_FOR_HOST needs a preceding space to separate it from
any other options that have already been set.

Signed-off-by: Michael Hope <michael.hope@linaro.org>

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}