gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19 (#145038)

This undoes a change made as a part of PR 137470, for compatibility with EMSDK
4.0.19. It adds `emscripten_trampoline` field in `pycore_runtime_structs.h`
and initializes it from JS initialization code with the wasm-gc based trampoline
if possible. Otherwise we fall back to the JS trampoline.