Fix unwinding on emscripten

[brson]

Today it's totally busted because, among other things, the libunwind symbols are unimplemented in emscripten. It may be worth having an unwinding path that uses libcxxabi for this target so that work doesn't need to be done in emscripten.

[brson]

I've started a patch to use the libcxxabi APIs on emscripten for unwinding.

[brson]

My utterly broken patch is https://github.com/brson/rust/tree/emscripten-unwind. Not sure I will continue to make progress on it, but it would be good to at least get it into a state where we can enable ERROR_ON_UNDEFINED_SYMBOLS #36515

[brson]

I've pushed a fix for this to #36339

[brson]

I've pushed a patch to #36339

[brson]

Fixed.