travis.yml: Test also the other targets on s390x

travis.yml: Test also the other targets on s390x

s390x is our only big endian host in our CI, so building and testing QEMU
there is quite valuable. Thus let's also test the other targets with
additional jobs (also using different sets of pre-installed libraries to
get a better coverage of the things that we test).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200608114049.4693-1-thuth@redhat.com>