[llvm] [libc][CMake] Fix build issues of libc on Windows (PR #102499)

[Schrodinger ZHU Yifan via llvm-commits]

https://github.com/SchrodingerZhu approved this pull request.

Another way as we tested is to add libc to `LLVM_ENABLE_RUNTIMES` and use `runtimes` as the source directory instead of `llvm`. 

https://github.com/llvm/llvm-project/pull/102499