testing/emscripten: upgrade to 1.38.34

Currently available version is from 2017.

Also, the homepage has changed to https://emscripten.org, and the source has also moved to https://github.com/emscripten-core/emscripten.

I reached out to the maintainer, but I never received a response.

I would be willing to submit a patch, I just don’t want to duplicate effort.

(from redmine: issue id 10549, created on 2019-06-10)