cmd/cgo: add support for GOARCH=arm64

Change-Id: Ia6c3d5e7a32b20e3c45d9485e66b48cd02644280
Reviewed-on: https://go-review.googlesource.com/7140Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/cgo/main.go

@@ -133,6 +133,7 @@ var ptrSizeMap = map[string]int64{
 	"386":     4,
 	"amd64":   8,
 	"arm":     4,
+	"arm64":   8,
 	"ppc64":   8,
 	"ppc64le": 8,
 	"s390":    4,
@@ -143,6 +144,7 @@ var intSizeMap = map[string]int64{
 	"386":     4,
 	"amd64":   8,
 	"arm":     4,
+	"arm64":   8,
 	"ppc64":   8,
 	"ppc64le": 8,
 	"s390":    4,