Add support for AVX2 instead of SSE2

Must compile with -mavx2 in clang/gcc or
 /arch:AVX2 in clang-cl

xxhash.h

@@ -5101,7 +5101,7 @@ XXH3_accumulate_512_avx2( void* XXH_RESTRICT acc,
                     const void* XXH_RESTRICT secret)
 {
     //DEBUG
-    printf("\nUsing AVX2\n");
+    // printf("\nUsing AVX2\n");
     XXH_ASSERT((((size_t)acc) & 31) == 0);
     {   __m256i* const xacc    =       (__m256i *) acc;
         /* Unaligned. This is mainly for pointer arithmetic, and because