Switch to a more proper matrix. Build each of the projects in a list
with DEBUG=0 and DEBUG=2 and with two versions of the toolchain (7.5.0
and 11.2.0).
Sets things up to be easier to run things like unit tests in a later
patch.
Use the new github actions to replace the travis-ci builders which
seem to no longer work and are being removed.
This first stab is fairly rough, just matches the explicit matrix
that was in the travis-ci hooks, but at least gets back to a buildable
state again.
