crypto/sha3: update from golang.org/x/crypto/sha3 a 80b25ed4

This pulls in the assembly implementation of keccakf1600 for amd64.

crypto/sha3/keccakf.go

@@ -2,6 +2,8 @@
 // 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 sha3
 
 // rc stores the round constants for use in the ι step.