Add GitHub Workflow for Linux (attempt MacOS)

[AE1020]

This triggers a GitHub Action that gets the dependencies for avr-gcc
(and also freeglut) in order to run the make on Linux.

After the make is complete, it runs the suggested test command from
the README.md. Currently this demonstrates that the output does
not match what is stated. This could be evolved into a test that
uses the continuous integration to enforce that the README.md output
stays in sync with the actual program output.

It includes an attempt to build on a Mac container. It gets an
error of "fatal: No names found, cannot describe anything"...which
someone familiar with MacOS who has seen a successful build would
be more qualified to try and fix than I would be.

[AE1020]

Here the Linux build is successfully installing dependencies and compiling:

https://github.com/AE1020/simavr/runs/2347149922?check_suite_focus=true#step:6:1

(Note: the MacOS build matrix line can be commented out if you don't offhand know how to fix it.)

But please see the output when running the example from README.md, which does not match the input.

output for $./simavr/run_avr tests/atmega88_example.axf

I mention in issue #168 that I wasn't getting the expected output. I figured that contributing a GitHub Action would be generically useful, as well as a good way to demonstrate that the disparity is not just on my machine.

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Be aware--if you are not already--that Travis has formally ceased providing CI credits for open source projects. Which is no big loss, as GitHub Actions is much better.