[llvm] Fix warnings while building llvm against emscripten for web assembly (PR #118262)

[via llvm-commits]

whitequark wrote:

@sbc100 Seems reasonable, although I will note that I've focused on building LLVM for bare WASI since I need it to run on both JS-ful and JS-less platforms. As such I've never really used Emscripten much.

https://github.com/llvm/llvm-project/pull/118262