cmd/gomobile: use ARC when compiling generated objective-c files.

Gobind -lang=objc generates code that assume use of ARC.
Set -fobjc-arc to explicitly enable ARC.

Change-Id: Ib0ec7b19773c112c01ed23cb00f1ec9d64946e6b
Reviewed-on: https://go-review.googlesource.com/14544
Reviewed-by: David Crawshaw <crawshaw@golang.org>

cmd/gomobile/bind_iosapp.go

@@ -118,6 +118,7 @@ func goIOSBindArchive(name, path string, env []string) (string, error) {
 		"-I", ".",
 		"-g", "-O2",
 		"-o", obj,
+		"-fobjc-arc", // enable ARC
 		"-c", "Go"+strings.Title(name)+".m",
 	)
 	cmd.Args = append(cmd.Args, strings.Split(getenv(env, "CGO_CFLAGS"), " ")...)