go/build: add GOARCH=arm64

Change-Id: I51db032e3dc2762d94e4000914b30813946250f7
Reviewed-on: https://go-review.googlesource.com/7044Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/go/build/build.go

@@ -1396,6 +1396,8 @@ func ArchChar(goarch string) (string, error) {
 		return "6", nil
 	case "arm":
 		return "5", nil
+	case "arm64":
+		return "7", nil
 	case "ppc64", "ppc64le":
 		return "9", nil
 	}

src/go/build/syslist.go

@@ -5,4 +5,4 @@
 package build
 
 const goosList = "android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows "
-const goarchList = "386 amd64 amd64p32 arm ppc64 ppc64le "
+const goarchList = "386 amd64 amd64p32 arm arm64 ppc64 ppc64le "