Add cmake Linux build and test job (#52)

Summary:
This adds a build and test workflow using CMake on Linux.  Here are some of my findings:

* The build only works if `executorch` is checked out into a directory called `executorch`.  Otherwise, including `<executorch/...>` would fail (https://github.com/pytorch/test-infra/pull/4465)
* Linking with GNU ld fails with the following error `/usr/bin/ld: -f may not be used without -shared`, which I'm not sure what it means atm, so I switch to LLD by adding `add_link_options("-fuse-ld=lld")` for clang
* I still couldn't make this work on Mac yet because `libprogram_schema.a` is empty and Mac `ar` refuses to link the library.  So that's an issue for another day.

```
/Library/Developer/CommandLineTools/usr/bin/ar qc libprogram_schema.a
ar: no archive members specified
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/52

Reviewed By: dbort

Differential Revision: D48216923

Pulled By: huydhn

fbshipit-source-id: 0ba751e3e849ec6779b19338253dd73b7ef0f3c8