Draft: emscripten: Use the latest emsdk 3.1.68

[kleisauke]

And make the changes in commit 63b0d67 unconditional.

Supersedes: #3569.
Cherry-picks a couple of changes from: #3307.
Depends on: rust-lang/rust#131533.

(untested, so opened as a draft)

[rustbot]

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[hoodmane]

Thanks @kleisauke it would be nice to see time_t finally get fixed...

[juntyr]

Could/should this PR keep the emcc version check to give a nice error at build time if a now-no-longer supported Emscripten is used?

[hoodmane]

I wouldn't want rust to generate an error when building with Emscripten 3.1.58 since that is the version that Pyodide is using with Python 3.12 and it would prevent us from upgrading. Though I suppose that it would just require us to use a patched rust/libc and we are already doing that, it would be nice to avoid additional patches.

[hoodmane]

But having a version check would be fine as long as there is an override.

[juntyr]

I was thinking about the previous check for 3.1.42 when the ABI change happened, which previous libc versions still supported (now there would be an ABI break in the opposite direction).

I definitely agree that other than that pretty much any emcc version has to be allowed since there is no current indicator which versions are ABI compatible and which aren’t