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 Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_SUFFIX:-${CT_ARCH_ALPHA_VARIANT}}"

}