gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19

[hoodmane]

This undoes a change made as a part of #137470. We add an emscripten_trampoline field in pycore_runtime_structs.h and initialize it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline.

cc @freakboy3742 @ryanking13

• Issue: Emscripten: Trampoline doesn't work with emcc 4.0.19 due to table/memory visibility #145037

[freakboy3742]

!buildbot emscripten

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit 526e102 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145038%2Fmerge

The command will test the builders whose names match following regular expression: emscripten

The builders matched are:

• WASM Emscripten PR

[freakboy3742]

@hoodmane Not sure what is going on here, but this has broken the Emscripten buildbot.

[hoodmane]

Looks like there are some mistakes to fix here.

[hoodmane]

!buildbot emscripten

I think it should be better now.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @hoodmane for commit f3d03d3 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145038%2Fmerge

The command will test the builders whose names match following regular expression: emscripten

The builders matched are:

• WASM Emscripten PR

[hoodmane]

Okay now it is better.