[Support/BLAKE3] quick fix for Cygwin build (#148635)
BLAKE3 1.8.2 ( imported in d2ad63a193216d008c8161879a59c5f42e0125cc )
fails to build for the Cygwin target.
see: https://github.com/BLAKE3-team/BLAKE3/issues/494
As a temporary workaround, add `&& !defined(__CYGWIN__)` to BLAKE3
locally.
resolves https://github.com/llvm/llvm-project/issues/148365
(cherry picked from commit 8de61eb01c9752f0488ed8b52d01fe3d0873ff6c)
