[Support/BLAKE3] quick fix for Cygwin build (llvm#148635)

BLAKE3 1.8.2 ( imported in d2ad63a )
fails to build for the Cygwin target.

see: BLAKE3-team/BLAKE3#494

As a temporary workaround, add `&& !defined(__CYGWIN__)` to BLAKE3
locally.

resolves llvm#148365

Author: kikairoya

llvm/lib/Support/BLAKE3/blake3_dispatch.c

@@ -236,7 +236,7 @@ void blake3_xof_many(const uint32_t cv[8],
 #if defined(IS_X86)
   const enum cpu_feature features = get_cpu_features();
   MAYBE_UNUSED(features);
-#if !defined(_WIN32) && !defined(BLAKE3_NO_AVX512)
+#if !defined(_WIN32) && !defined(__CYGWIN__) && !defined(BLAKE3_NO_AVX512)
   if (features & AVX512VL) {
     blake3_xof_many_avx512(cv, block, block_len, counter, flags, out, outblocks);
     return;

llvm/lib/Support/BLAKE3/blake3_impl.h

@@ -324,7 +324,7 @@ void blake3_hash_many_avx512(const uint8_t *const *inputs, size_t num_inputs,
                              uint8_t flags, uint8_t flags_start,
                              uint8_t flags_end, uint8_t *out);
 
-#if !defined(_WIN32)
+#if !defined(_WIN32) && !defined(__CYGWIN__)
 LLVM_LIBRARY_VISIBILITY
 void blake3_xof_many_avx512(const uint32_t cv[8],
                             const uint8_t block[BLAKE3_BLOCK_LEN],