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

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whitequark wrote:

> Alternatively, perhaps it would be good to stub it out at the pthread API level, so that C code is also handled.

This is [being discussed in wasi-libc](https://github.com/WebAssembly/wasi-libc/issues/501#issuecomment-2227444912) and might be a good way forward.

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