[dev.power64] hash/crc32: use portable castagnoli on power64

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/128110044

src/pkg/hash/crc32/crc32_generic.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 arm
+// +build 386 arm power64 power64le
 
 package crc32