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

[via llvm-commits]

jeremyd2019 wrote:

@mstorsjo 
I'm not sure about the `_FILE_OFFSET_BITS` define - I don't see that Cygwin uses it to change the size of `off_t`, so I could add `LLVM_USING_GLIBC` to its `if`, but I also don't see how it would hurt anything, and 32-bit Cygwin is barely on life support at this point anyway.

(I only plan to try to support it until Windows 10 end-of-support, and Git-for-Windows needs to support some tiny subset until 2029 IIRC, and I'm sure llvm isn't part of that since it hasn't been usable up to now).

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