[3.14] gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19 (GH-145038) (#145283)

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.
(cherry picked from commit 43fdb7037e76c18d9545ac11b2f1e3e398152ada)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>