.travis.yml: Shorten the runtime of the problematic jobs

.travis.yml: Shorten the runtime of the problematic jobs

The "[s390x] GCC (other-system)" and the "[s390x] GCC check-tcg"
jobs are hitting the 50 minutes timeout in Travis quite frequently
since a while.

To fix it, we've got to drop a lot of the targets from the target
list in the jobs to make them work again.

With regards to the "check-tcg" test, we can move the check with
"s390x-linux-user" to the "user" job instead which also builds
the s390x-linux-user target.

And while we're at it, remove the "--enable-fdt=system" configure
switch (since this is not required nowadays anymore).

Message-ID: <20240320104144.823425-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>