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

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EricWF wrote:

@trcrsired Could you please describe what bits of libunwind WASM uses? It seems to me almost all of the functionality is stripped out?

Could you also provide instructions to build and test libunwind under WASM so we could add CI coverage? And so I can test this change myself.

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