Expected Behavior

Dune 3.20.0 should compile successfully on Cygwin/Windows x86_64 systems during the bootstrap
process.

Actual Behavior

Dune 3.20.0 compilation fails during the bootstrap phase with linking errors related to
unresolved Blake3 symbols:

** Cannot resolve symbols for blake3_dispatch.o:
[...]
blake3_compress_xof_avx512
blake3_compress_xof_sse2
blake3_compress_xof_sse41
blake3_hash_many_avx2
blake3_hash_many_avx512
blake3_hash_many_sse2
blake3_hash_many_sse41
blake3_xof_many_avx512
File "caml_startup", line 1:
Error: Error during linking (exit code 2)

Reproduction

This occurs when trying to install any package that depends on Dune in a fresh opam environment
on Cygwin/Windows:

1. Initialize opam with a recent opam-repository commit: opam init --bare default
   git+https://github.com/ocaml/opam-repository.git#d87894f816bea883931937e050bb5be4f108e50a
2. Create a switch with system OCaml: opam switch create default ocaml-system
3. Try to install any package requiring Dune: opam install lwt

see https://github.com/arozovyk/opam/actions/runs/17233978247/job/48894396963

Specifications

• Version of dune 3.20.0 (from opam-repository#d87894f816bea883931937e050bb5be4f108e50a)
• Version of ocaml 4.14.2
• Operating system (distribution and version): Cygwin on Windows x86_64

related to #11735 (cygwin is not tested by the CI)