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

[via cfe-commits]

whitequark wrote:

I've updated the PR to use almost exclusively `defined(__wasi__)`/`defined(__wasm__)`, with the exception of `HAVE_SETJMP`, where you could conceivably have both kinds of builds on WebAssembly depending on your needs.

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