[libunwind] [libunwind][WebAssembly] Omit unused parts of libunwind.cpp for Wasm (PR #73196)

[Heejin Ahn via cfe-commits]

aheejin wrote:

@trcrsired When we use it in Emscripten, we build it with `-D__USING_WASM_EXCEPTIONS__`. Does that work? I'm not sure which toolchain and libraries you are using though. (It looks you are using WASI, but does WASI have the EH support? Does it allow usage with llvm libunwind?)

https://github.com/llvm/llvm-project/pull/73196