[Support/BLAKE3] #147948 breaks Cygwin build

[kikairoya]

#147948 causes breaking of Cygwin build.

/home/kikai/llvm-origin-main/llvm/lib/Support/BLAKE3/blake3_dispatch.c: 関数 ‘blake3_xof_many’ 内:
/home/kikai/llvm-origin-main/llvm/lib/Support/BLAKE3/blake3_dispatch.c:241:5: エラー: implicit declaration of function ‘blake3_xof_many_avx512’; did you mean ‘blake3_hash_many_a  avx512’? [-Wimplicit-function-declaration]
  241 |     blake3_xof_many_avx512(cv, block, block_len, counter, flags, out, outblocks);
      |     ^~~~~~~~~~~~~~~~~~~~~~
      |     blake3_hash_many_avx512

I've reported for upstream: BLAKE3-team/BLAKE3#494

There are possibly workarounds:

1. Patch blake3 itself
2. Use blake3_avx512.c instead of assembly on Cygwin
3. Define _WIN32 for compiling blake3_dispatch.c

I'm not sure which of these approaches would be the most appropriate, so I'd appreciate any suggestions.
Once there's a consensus, I'm happy to submit a PR accordingly.

[Meinersbur]

Should do the same as upsteam does.

For quick build fix, BLAKE3-team/BLAKE3#496 looks fine to apply locally.