Enable WASM build on GitHub Actions

[jiepan-intel]

When building XNNPACK with Emscripten, I often meet build warning, enable WASM build on Github Actions may solve this issue. single-threaded WASM test can also run under d8(developer shell of v8), but it requires d8 binary, create build job without test job is enough to resolve build issue. Does XNNPACK plan to support this? Thanks!

[alankelly]

Hi, a pull request for this would be most welcome! Probably involves editing: https://github.com/google/XNNPACK/blob/master/.github/workflows/build.yml

Thanks!