[libunwind] [libunwind] Exlcude wasm build from using __declspec export (PR #168449)

[Alex Crichton via cfe-commits]

alexcrichton wrote:

I wanted to ping here and ask -- was there anything remaining for this? I think this is the last piece needed for C++ exceptions in wasi-sdk, so I was curious to ask about and possibly help get this landed.

https://github.com/llvm/llvm-project/pull/168449