[wasm-split] Inbuilt instrumentation support with threads enabled

Currently, emscripten instruments wasm binaries without using `--in-memory` or `--in-secondary-memory` wasm-split options when threads are enabled [here](https://github.com/emscripten-core/emscripten/blob/main/tools/link.py#L466). Even [docs](https://emscripten.org/docs/optimizing/Module-Splitting.html#multithreaded-programs) mention using `--in-memory` for multithreaded programs. I have few questions regarding this:

1. Do we need to re-instrument the wasm binary ourselves for multithreaded programs? Or has emscripten planned to provide support for this?
2. Docs also suggest to use `-sGLOBAL_BASE` flag with `--in-memory` wasm-split option. What about `--in-secondary-memory` option? Do we still need to use that flag if we use the later option?