[iOS] Always pass "-ObjC" when linking iOS executables.

Chrome on iOS needs to use "-ObjC" to link all symbols from static
libraries that define at least one Objective-C category or class.
Add it to the ios_app_bundle template so that it is used by all
executables.

BUG=459705

Review-Url: https://codereview.chromium.org/2024673002
Cr-Commit-Position: refs/heads/master@{#397087}

build/config/ios/rules.gni

@@ -141,6 +141,10 @@ template("ios_app_bundle") {
       libs = []
     }
     libs += [ "UIKit.framework" ]
+    if (!defined(ldflags)) {
+      ldflags = []
+    }
+    ldflags += [ "-ObjC" ]
   }
 
   bundle_data(_bundle_data_executable) {