Check with GitHub Actions

[krlmlr]

• Ubuntu 16.04 and 18.04
• macOS (checks failing)
• mingw32 and mingw64 (checks failing)

If this PR is merged, the checks will look like https://github.com/krlmlr/libbacktrace/actions/runs/231224462 and will run for every commit to the main branch and for every pull request.

[cooljeanius]

actionlint says:

$ actionlint
.github/workflows/check.yaml:35:15: the runner of "actions/checkout@v2" action is too old to run on GitHub Actions. update the action's version to fix this issue [action]
   |
35 |       - uses: actions/checkout@v2
   |               ^~~~~~~~~~~~~~~~~~~