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.

[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.