We have to use "debian.jessie" instead of "ubuntu.xenial" because
qemu-s390x-static can only run z900 instructions and Ubuntu is
built with a more recent architecture level set.

We first enable only gcc because there are still some issues with clang.
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>