asyncify - option to use multiple memories

[wanderer]

There are a few implementation of the nascent multiple memory proposal now. Asyncify could benefit from using a dedicated memory, which would simplify the api since you wouldn't need to specify the memory offset where unwind data lives.

[tlively]

cc @ashleynh

[L-jasmine]

I did it https://github.com/WebAssembly/binaryen/pull/4853