docker/dockerfiles/debian-s390-cross: include clang

It's a silly little limitation on Shippable that is looks for clang
in the container even though we won't use it. The arm/aarch64 cross
builds inherit this from debian.docker but as we needed to use
debian-testing for this we add it here. We also collapse the update
step into one RUN line to remove and intermediate layer of the docker
build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170306112848.659-1-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
