ci: use an explicit list of Travis jobs

ci: use an explicit list of Travis jobs

Maintaining the .travis.yml requires some knowledge of how Travis
computes the jobs list (combination of os: arch: compiler: etc...).
Let's switch to an explicit list to find all jobs at a glance.

To enhance readability, jobs have been sorted per arch/compiler with
comments to isolate blocks.

Setting required_packages for aarch64 native jobs is unnecessary,
the global addons: values are the same.

This commit does not change the jobs list (21 jobs in total).

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Aaron Conole <aconole@redhat.com>