gh-145177: Bump emscripten to 5.0.4

[hoodmane]

Before Emscripten 4.0.19, there was a bug in the dynamic loader that caused any
dynamic library that links libffi to fail to load. _ctypes_test.so unnecessarily
links libffi so it would fail to load and tests that needed it were skipped.

There are two test failures behind that: one involving stack overflows which
we have to skip as usual, and one that assumes that the abi for a function
that takes a single struct with two doubles is the same as the abi for a
function that takes two double arguments. This is not true in webassembly so we
skip the test.

There is another regression in Emscripten 5.0.5 which breaks a couple tests in
test_secrets and test_random, so I'm leaving updating past that to a followup.

Ideally #151423 should merge before this.

• Issue: Emscripten buildbot: Support multiple Emscripten versions #145177

[hoodmane]

Okay there are still a few other test failures to investigate here.

[freakboy3742]

This all makes sense, once (as you've noted) #151423 has landed.

The only question I have is about the backport strategy (and, I guess, the wider strategy about when the cutoff for Emscripten version bumps actually is). Should this be back ported to the 3.15 branch? The beta is the point at which binary and byte code compatibility is guaranteed for any other platform; Emscripten version compatibility would seem to be in the same boat - except that binary compatibility in practice is really determined by the PEP 783 pyemscripten tag.

However, pyemscripten_2026_0 (matching 3.14) is still listed as "in development", and pyemscripten_2027_0 (which would match 3.15) isn't listed at all, which would seem to suggest backporting this won't cause incompatibilities.

So - from Pyodide's perspective, when is the pyemscripten_X_0 tag finalised?

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[hoodmane]

pyemscripten_2026_0 is now frozen at 5.0.3 whereas the cpython 3.14 branch is on 4.0.12. I think it's a separate task we have to do to sync these up. Pep 783 being accepted clears a major blocker, and I think it'd be nice if we made a Pyodide release with Python 3.15 soon after Python 3.15 itself is released. We will have to discuss a bit more with the other Pyodide maintainers cc @ryanking13

But it would be nice if Python 3.15 can be based on Emscripten 6.0.0 or later because until version 6.0.0 Emscripten ignored the -shared flag but from now on it respects it. This should allow a simplification of our cross build system.