Fix GN os, cpu defaults for Android builds.

tools/relocation_packer/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.
 
-relocation_packing_supported = target_arch == "arm" || target_arch == "arm64"
+relocation_packing_supported = target_cpu == "arm" || target_cpu == "arm64"
 
 if (relocation_packing_supported) {
   relocation_packer_target = "//tools/relocation_packer($host_toolchain)"
   relocation_packer_dir =
       get_label_info("$relocation_packer_target", "root_out_dir")
   relocation_packer_exe = "${relocation_packer_dir}/relocation_packer"
 
-  if (target_arch == "arm") {
+  if (target_cpu == "arm") {
     relocations_have_addends = 0
-  } else if (target_arch == "arm64") {
+  } else if (target_cpu == "arm64") {
     relocations_have_addends = 1
   }
 } else {
   relocations_have_addends = 0
   relocation_packer_exe = ""
 }