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

[via llvm-commits]

jeremyd2019 wrote:

so
```cmake
if (CYGWIN)
  add_compile_definitions(_GNU_SOURCE)
  list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
endif ()
```
right above, to go with the `SunOS` and `AIX` checks, and leave the `LLVM_USING_GLIBC` if alone?

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