crypto/sha512: avoid duplicate block declaration on 386

Unbreak the build - we do not have a sha512 block implementation in
386 assembly (yet).

R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/48520043

src/pkg/crypto/sha512/sha512block_decl.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build 386 amd64
+// +build amd64
 
 package sha512