[iOS] Define build/config/ios:ios_dynamic_flags for default linker flags.

build/config/ios/rules.gni

Index: build/config/ios/rules.gni
diff --git a/build/config/ios/rules.gni b/build/config/ios/rules.gni
index b0d248b107cc6b56e74b5ef0d59e06e49481372f..30cf52c64b5dc0eb912801f16dad758d1bccf277 100644
--- a/build/config/ios/rules.gni
+++ b/build/config/ios/rules.gni
@@ -176,10 +176,6 @@ template("ios_app_bundle") {
         libs = []
       }
       libs += [ "UIKit.framework" ]
-      if (!defined(ldflags)) {
-        ldflags = []
-      }
-      ldflags += [ "-ObjC" ]
     }
   } else {
     # This is either a thin build or the default toolchain of a fat-build.
@@ -263,10 +259,6 @@ template("ios_app_bundle") {
         libs = []
       }
       libs += [ "UIKit.framework" ]
-      if (!defined(ldflags)) {
-        ldflags = []
-      }
-      ldflags += [ "-ObjC" ]
     }
 
     if (_is_fat_build) {