update the blake2b implementation (#3724)

Fixes a performance bug caused by SSE-AVX register savings on amd64.

vendor/golang.org/x/crypto/blake2b/blake2b_ref.go

@@ -0,0 +1,11 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !amd64 appengine gccgo
+
+package blake2b
+
+func hashBlocks(h *[8]uint64, c *[2]uint64, flag uint64, blocks []byte) {
+	hashBlocksGeneric(h, c, flag, blocks)
+}