[clang] [libcxxabi] [llvm] Add support for WASI builds (PR #91051)

[Louis Dionne via cfe-commits]

https://github.com/ldionne approved this pull request.

FWIW I am happy with the one line change in `libcxxabi/`. For libc++ to officially support this beyond just a fun proof of concept, we'd need a story for running the tests and set up a pre-commit CI for it though.

I'm approving just to get the `reviewers-libcxxabi` group out of your way, I didn't look at the rest of the PR. Also, if you think we should go ahead with the other PR instead and don't plan on merging this, please close this PR to help us keep our PR queue tidy. Thanks!

https://github.com/llvm/llvm-project/pull/91051