travis.yml: Improve the --disable-tcg test on s390x

Since the s390x containers do not allow KVM, we only compile-test
the --disable-tcg build on s390x and do not run the qtests. Thus,
it does not make sense to install genisoimage here, and it also does
not make sense to build the s390-ccw.img here again - it is simply
not used without the qtests.
On the other hand, if we do not build the s390-ccw.img anymore, we
can also compile with Clang - so let's use that compiler here to
get some additional test coverage.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200512133849.10624-1-thuth@redhat.com>
Message-Id: <20200513175134.19619-3-alex.bennee@linaro.org>