os/signal, hash/crc32: add arm64 build tags

Change-Id: I6ca9caec8ccf12618e56dcf6b83328e7acf8b1ec
Reviewed-on: https://go-review.googlesource.com/7148Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Russ Cox <rsc@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 ppc64 ppc64le
+// +build 386 arm arm64 ppc64 ppc64le
 
 package crc32
 

src/os/signal/sig.s

@@ -4,7 +4,7 @@
 
 // Assembly to get into package runtime without using exported symbols.
 
-// +build amd64 amd64p32 arm 386 ppc64 ppc64le
+// +build amd64 amd64p32 arm arm64 386 ppc64 ppc64le
 
 #include "textflag.h"