Enable Wasm BigInt. This enables WASM_BIGINT while maintaining (hypothetical) Safari 14 support
by shimming BigInt64Array and BigUint64Array if they are missing. I think the
last time we tried to enable WASM_BIGINT was before #2019 so our chances
are significantly better this time.

This will fix dynamic linking bugs, reduce code size, and improve speed.

See also my upstream PR:
https://github.com/emscripten-core/emscripten/pull/17103

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
