hash/crc32: added mips64{,le} build tags

Change-Id: I77c6768fff6f0163b36800307c4d573bb6521fe5
Reviewed-on: https://go-review.googlesource.com/14454Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/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 arm64 ppc64 ppc64le
+// +build 386 arm arm64 mips64 mips64le ppc64 ppc64le
 
 package crc32