travis.yml: Run tests on the non-x86 builders, too

Travis recently added the possibility to compile on aarch64, ppc64le
and s390x hosts, so let's add this possibility to the dtc CI, too.
Unfortunately, there are some weird valgrind errors when running
on ppc64le (which rather look like a problem on the valgrind side to
me, and not in dtc), so we can not use "checkm" on ppc64le yet.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191203122020.14442-1-thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>