Add `pyodide xbuildenv install-emscripten` command

[ryanking13]

This issue proposes to add a new subcommand under pyodide xbuildenv that installs emscripten.

This has been discussed a lot before, but we haven't been providing this command, as we thought it was not our responsibility to install emscripten.

However, as we often have Emscripten patches that affect Package build, I think it would be reasonable to provide a subcommand under pyodide-build to install emscripten and apply patches.

My proposal is to add it under the xbuildenv subcommand, say pyodide xbuildenv install-emscripten. This command should install and activate the emscripten inside the xbuildenv directory (Note that we have a subdirectory per pyodide version).

We also need to provide a way to automatically add the installed emscripten to PATH. Otherwise, we can provide another subcommand to show the path to theemsdk_env.sh so that user can activate it manually in their shell.

[bulenty584]

Hey @ryanking13. You mind if I tackle this?

[bulenty584]

The only caveat I have is where we would pull patches from during the installation process. Maybe a subcommand to wget the patches during this process? I'm assuming the command can't assume presence of the patches that are in the larger pyodide dist.?

[ryanking13]

@bulenty584 Sure, thanks.

The only caveat I have is where we would pull patches from during the installation process. Maybe a subcommand to wget the patches during this process?

We deploy emscripten patches in our cross-build env archive since Pyodide 0.28. You don't need to download patches separately. Please check the script that we are using in pyodide-recipes directory to install and patch emscripten.