vendor x/sys/cpu and update x/crypto/blake2b (#5870)

This change updates the vendor'd x/crypto/blake2b package and
adds x/sys/cpu.

This change fixes an issue for Go1.11 affecting x/crypto/blake2b
since G1.11 removes the runtime-internal functions `suports_avx()` and
`supports_avx2()`.

vendor/golang.org/x/sys/cpu/cpu_mipsx.go

@@ -0,0 +1,9 @@
+// Copyright 2018 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 mips mipsle
+
+package cpu
+
+const cacheLineSize = 32