[llvm] [ci] Add Emscripten workflow (PR #151928)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> This looks like a niche configuration that should be covered by a buildbot, not pre-commit CI.

Agreed. Using Mac/Windows runners for almost two hours per PR I'm pretty sure is untenable. Configurations like this are also canonically tested postcommit.

https://github.com/llvm/llvm-project/pull/151928