crypto/md5: always test the portable block function too

So it doesn't bitrot.

Like the sha1 version (https://golang.org/cl/62270043)

LGTM=agl
R=agl
CC=golang-codereviews
https://golang.org/cl/62420043

src/pkg/crypto/md5/gen.go

@@ -167,8 +167,6 @@ var program = `// Copyright 2013 The Go Authors. All rights reserved.
 // DO NOT EDIT.
 // Generate with: go run gen.go{{if .Full}} -full{{end}} | gofmt >md5block.go
 
-// +build !amd64,!386,!arm
-
 package md5
 
 import (
@@ -204,7 +202,7 @@ func init() {
 	littleEndian = *(*[4]byte)(unsafe.Pointer(&x)) == y
 }
 
-func block(dig *digest, p []byte) {
+func blockGeneric(dig *digest, p []byte) {
 	a := dig.s[0]
 	b := dig.s[1]
 	c := dig.s[2]

src/pkg/crypto/md5/md5_test.go

@@ -5,6 +5,7 @@
 package md5
 
 import (
+	"crypto/rand"
 	"fmt"
 	"io"
 	"testing"
@@ -105,6 +106,18 @@ func TestLarge(t *testing.T) {
 	}
 }
 
+// Tests that blockGeneric (pure Go) and block (in assembly for amd64, 386, arm) match.
+func TestBlockGeneric(t *testing.T) {
+	gen, asm := New().(*digest), New().(*digest)
+	buf := make([]byte, BlockSize*20) // arbitrary factor
+	rand.Read(buf)
+	blockGeneric(gen, buf)
+	block(asm, buf)
+	if *gen != *asm {
+		t.Error("block and blockGeneric resulted in different states")
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192+1)
 var sum = make([]byte, bench.Size())

src/pkg/crypto/md5/md5block.go

@@ -5,8 +5,6 @@
 // DO NOT EDIT.
 // Generate with: go run gen.go -full | gofmt >md5block.go
 
-// +build !amd64,!386,!arm
-
 package md5
 
 import (
@@ -24,7 +22,7 @@ func init() {
 	littleEndian = *(*[4]byte)(unsafe.Pointer(&x)) == y
 }
 
-func block(dig *digest, p []byte) {
+func blockGeneric(dig *digest, p []byte) {
 	a := dig.s[0]
 	b := dig.s[1]
 	c := dig.s[2]

src/pkg/crypto/md5/md5block_generic.go

+// Copyright 2014 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,!386,!arm
+
+package md5
+
+var block = blockGeneric