[3.14] gh-145177: Support multiple Emscripten versions for Emscripten buildbot (GH...

[3.14] gh-145177: Support multiple Emscripten versions for Emscripten buildbot (GH-145180) (#145582)

Adds an `--emsdk-cache` argument to the Emscripten build script and an
emscripten_version.txt file. If the `--emsdk-cache` argument is passed, the build script
will look in `emscripten_version.txt` to get the expected emsdk version is installed in
a folder called e.g., 4.0.12 in the directory indicated by the `--emsdk-cache` argument,
and run the build with that Emscripten tooling activated.
(cherry picked from commit c3fb0d9d96902774c08b199dda0479a8d31398a5)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>