[libunwind] [libunwind][WebAssembly] Fix libunwind.cpp guard (PR #78230)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> This should have been &&, meaning neither SjLj nor Wasm uses this file.

Thanks for the description. In the future, it'd be better to reference what PR this PR fixes.

https://github.com/llvm/llvm-project/pull/78230