hash/crc32: make compatible with go/build

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5486060

src/pkg/hash/crc32/crc32_generic.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 386 arm
+
 package crc32
 
 // The file contains the generic version of updateCastagnoli which just calls