scripts: fix finishing the toolchain when download/extract-only is set

In case we only download or extract the sources, do not fail while
finishing the toolchain: the test-suite directory may not exist, so
we can't chmod it.

Also, use safer constructs that won't trigger the 'set -e' in case of
failure (eg.: "[ ... ] && ..." is not safe in case the test fails).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x${CT_ARCH_SUFFIX}"

    fi

}