[clang] [llvm] Conditionalize use of POSIX features missing on WASI/WebAssembly (PR #92677)

[via cfe-commits]

whitequark wrote:

Now that there are [instructions](https://discourse.llvm.org/t/rfc-building-llvm-for-webassembly/79073/36?u=whitequark) for trying the Wasm-hosted build, and I've verified that it works on relatively simple examples, I think this PR can progress forward.

I can see the buildbot failures; I'm not yet sure what's up with them, any pointers are appreciated.

https://github.com/llvm/llvm-project/pull/92677