[Android] Print unknown architecture name in gn error message.

build/config/android/config.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file contains common system config stuff for the Android build.
 
 # TODO(brettw) bug 341772 put this into the is_android block when the toolchain
 # stuff is fixed in the GYP generator. The problem is that when we redo the
 # build in host mode, the OS is not android and the declarations are never
 # seen, which throws an error because these arguments are specified.
@@ skipping 37 matching lines @@
   } else if (cpu_arch == "arm") {
     import("//build/config/arm.gni")
     if (arm_version < 7) {
       android_app_abi = "armeabi"
     } else {
       android_app_abi = "armeabi-v7a"
     }
   } else if (cpu_arch == "mipsel") {
     android_app_abi = "mips"
   } else {
-    assert(false, "Unknown Android ABI")
+    assert(false, "Unknown Android ABI: " + cpu_arch)
   }
 } else {
   if (!defined(is_android_webview_build)) {
     is_android_webview_build = false
   }
   use_system_stlport = false
 }