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

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whitequark wrote:

I've further confirmed that clang.wasm can build complex software by building [Yosys](https://github.com/yosysHQ/yosys/) for Wasm using clang.wasm. It works fine and is able to translate a simple design; this makes it clear that a mature codebase with a mixture of C and C++ code can compile, link, and produce a working artifact using this port, so I have a high degree of confidence in this PR now.

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