[compiler-rt] [libcxx] [libcxxabi] [libunwind] [libunwind][libcxx][libcxxabi][compiler-rt-builtins] Fix Exception Handling build for wasm (PR #79667)

[Louis Dionne via cfe-commits]

https://github.com/ldionne requested changes to this pull request.

I think support for wasm mandates a short RFC, like most projects to support new platforms that require non-trivial changes. There's not a huge amount of changes in this patch, but it's also unclear to me that this is sufficient to do anything.

In particular, the following should be covered:
- What are the particularities of this platform, if any
- Who is supporting this platform and what is the expected support story
- What is the story for testing. How do you run tests on that platform? Do we need to make changes to the libc++ conformance test suite for things to work?

That would be the minimum. We have a policy of not accepting changes to add support for new platforms for platforms that are not officially supported. It would be amazing to support wasm and I'm sure it can be done, but we need to do it the right way, not only whip out a patch and call it a day.

I know we've accepted some isolated wasm changes in the past so this may sound surprising, however given the scope of the changes here I think this deserves to be done properly.

https://github.com/llvm/llvm-project/pull/79667