cmd/gomobile: use arm64 at packagesConfig

Go 1.15 no longer supports darwin/arm. arm64 is still supported by
Android and iOS.

Updates golang/go#39575

Change-Id: I91f09477ae68de148e4d63eef38d7d28f553d16c
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/241717
Run-TryBot: Hajime Hoshi <hajimehoshi@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/bind.go

@@ -215,7 +215,7 @@ func writeFile(filename string, generate func(io.Writer) error) error {
 func packagesConfig(targetOS string) *packages.Config {
 	config := &packages.Config{}
 	// Add CGO_ENABLED=1 explicitly since Cgo is disabled when GOOS is different from host OS.
-	config.Env = append(os.Environ(), "GOARCH=arm", "GOOS="+targetOS, "CGO_ENABLED=1")
+	config.Env = append(os.Environ(), "GOARCH=arm64", "GOOS="+targetOS, "CGO_ENABLED=1")
 	tags := buildTags
 	if targetOS == "darwin" {
 		tags = append(tags, "ios")