[Fixed] Specify to clang to use x86_64 to build models and tests for ObjectiveCxx

Summary:
Fix the issue with Linux, introduced in 721cc1957c and temporarily fixed in aef13134bf
Now clang gets the `x86_64-apple-darwin14` target only when running on ObjCXX source code with a specific `isysroot` flag.

infer/tests/utils/InferRunner.java

@@ -191,7 +191,8 @@ public class InferRunner {
       isysrootOption
           .add("-isysroot")
           .add(isysroot)
-          .add("-mios-simulator-version-min=8.2");
+          .add("-mios-simulator-version-min=8.2")
+          .add("--target=x86_64-apple-darwin14");
     }
     ImmutableList.Builder<String> arcOption =
         new ImmutableList.Builder<>();