[libc][CI] unanticipated macro expansion of clearerr_unlocked in macos-15 build

See https://github.com/llvm/llvm-project/actions/runs/20103522181/job/57680755758

Macos sdk defines clearerr_unlocked as a macro and [file.h](https://github.com/llvm/llvm-project/blob/main/libc/src/__support/File/file.h) defines it as a function. When running on macos, the preprocessor erroneously substitutes the defintion for the macro in place of the function identifier causing build failures.