Fix modules_unittests on iOS.

webrtc/modules/BUILD.gn

 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../build/webrtc.gni")
 import("audio_coding/audio_coding.gni")
@@ skipping 649 matching lines @@
       shard_timeout = 900
     }
     if (is_ios) {
       deps += [ ":modules_unittests_bundle_data" ]
       configs += [
         "..:common_objc",
         "//build/config/compiler:enable_arc",
       ]
 
       sources += [
-        "audio_device/ios/audio_device_unittest_ios.cc",
         "audio_device/ios/objc/RTCAudioSessionTest.mm",
         "video_coding/codecs/h264/h264_video_toolbox_nalu_unittest.cc",
       ]
 
+      if (target_cpu != "x64") {
+        sources += [ "audio_device/ios/audio_device_unittest_ios.cc" ]
+      }
+
       ldflags = [ "-ObjC" ]
     }
   }
 }