ci: reorganise Travis jobs

Let's prune the jobs list to limit the amount of time spent by the robot
in Travis.

Since meson enables automatically the relevant components, there is not
much gain in testing with extra_packages vs required_packages.

For a given arch/compiler/env combination, compilation is first tested
in all jobs that run tests or build the docs or run the ABI checks.
In the same context, for jobs that accumulates running tests, building
the docs etc..., those steps are independent and can be split to save
some cpu on Travis.

With this, we go down from 21 to 15 jobs.

Note: this patch requires a flush of the existing caches in Travis.

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