Improvements to Emscripten build script

[hoodmane]

@freakboy3742's desired improvements to the Emscripten build script:

1. clean should be a little more selective. It currently destroys all of cross-build; it should only be purging the parts of cross-build that are emscripten specific (so that Emscripten, iOS and Android builds can co-exist). This is admittedly a "me" problem... but it's a problem I have :-)
2. The cross-build directory name should be configurable. Again, this is somewhat a "me" problem... but I currently need to have cross builds for 3.10-3.15, and they can't currently co-exist.
3. We shouldn't be building dependencies (libFFI and mpdec) on every build. A similar caching strategy to the one described for EMSDK should be possible - look to this directory; if the cache has a build for the right emscripten version, use it rather than rebuilding.
4. Add install-emscripten subcommand

Linked PRs

• gh-145219: Emscripten bld configurable directory & only clean Emscripten #145581
• [3.14] gh-145219: Add Emscripten cross-build and clean configurability (GH-145581) #145654
• gh-145219: Cache Emscripten libffi and mpdec builds, add install-emscripten cmd #145664
• [3.14] gh-145219: Cache Emscripten build dependencies, add install-emscripten (GH-145664) #145790

[hoodmane]

@freakboy3742 I can make these changes but I want to wait until #145177 is merged to avoid conflicts.

[hoodmane]

I pushed fixes for 1 and 2 on https://github.com/hoodmane/cpython/tree/configurable-cross-build-dir on top of #145177.

[hoodmane]

@freakboy3742 for caching libraries, I'm thinking maybe we do the following:

1. Move emsdk from <emsdk-cache>/<version>/emsdk_env.sh to <emsdk-cache>/<version>/emsdk/emsdk_env.sh
2. If --emsdk-cache is provided, put the prefix at <emsdk-cache>/<version>/prefix.

Sound reasonable to you?

[freakboy3742]

Yes, I think that makes sense.

An additional suggestion: For consistency with other builds, we should simplify the <cross-build>/wasm32-emscripten/python directory to just be the Python build (i.e., build Python in <cross-build>/wasm32-emscripten), use <emsdk-cache>/<version>/mpdecimal-x.y.z as the location for a build for mpdecimal if a cache is provided, or <cross-build>/dependencies/wasm32-emscripten/mpdecimal-x.y.z if it isn't, and shortcut dependency builds if that directory already exists.

That would allow build artefacts for mpdecimal and libffi to persist between builds, increases consistency between Android, iOS and Emscripten build structures, and in particular makes the cleaning strategy consistent.

(I guess strictly we should be looking for one of the the installed products in <prefix> rather than the build directory as the shortcut trigger; I'm not sure if it will make that much difference in practice)