[llvm] [Cygwin] Don't use version script for Cygwin target (PR #143133)

[via llvm-commits]

jeremyd2019 wrote:

It looks like `LLVM_HAVE_LINK_VERSION_SCRIPT` is used in `libclang` and `AddLLVM.cmake` but not in `clang-shlib` or `llvm-shlib`

https://github.com/llvm/llvm-project/pull/143133