Add github actions CI inspired by travis/appveyor scripts

[bernhardmgruber]

This PR adds a few CI builds on Github actions that I have put together form a CI file of my own and taking inspiration from the existing travis/appveyor CI scripts.

[bernhardmgruber]

I think we need to decide which compilers we care about: https://github.com/bernhardmgruber/Vc/actions/runs/961463706
VS2019 and ipcx fail the tests because their results diverge from the reference results more ULPs than other compilers. We could fix that, remove those compilers from the CI or just flag those CI jobs as not mandatory for PRs.

[amadio]

We need to add -fp-model=precise for ICC and related compilers, I think. They are by default a bit too aggressive with unsafe arithmetic/math optimizations.

[bernhardmgruber]

We need to add -fp-model=precise for ICC and related compilers, I think.

Good guess. Vc detects icpc and icc and sets -fp-model source. That could solve icpx.
The Visual Studio 2019 builds are still failing though.

[bernhardmgruber]

I removed the failing CI runs for now and will create a separate PR for them later.

[agheata]

Looks good!