tests/tcg: enable building for s390x

This doesn't add any additional tests but enables building the
multiarch tests for s390x.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Acked-by: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>

tests/tcg/s390x/Makefile.include

+DOCKER_IMAGE=debian-s390x-cross
+DOCKER_CROSS_COMPILER=s390x-linux-gnu-gcc