[llvm] [LLVM][Cygwin] Define _GNU_SOURCE on Cygwin as well. (PR #138329)

[via llvm-commits]

jeremyd2019 wrote:

cygwin's packaging builds clang standalone, and in that configuration it seems that `_GNU_SOURCE` is not defined despite this change.  Any thoughts?

https://github.com/llvm/llvm-project/pull/138329