RandomX: added Blake2 AVX2 version

+0.1% speedup on AMD Zen2/Zen3 and Intel CPUs which support AVX2.

src/crypto/randomx/blake2/avx2/blake2b.h

@@ -0,0 +1,16 @@
+#ifndef BLAKE2_AVX2_BLAKE2B_H
+#define BLAKE2_AVX2_BLAKE2B_H
+
+#include <stddef.h>
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+int blake2b_avx2(void* out, size_t outlen, const void* in, size_t inlen);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif