GN: make relocation packing work

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"))
+
+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") {
+    relocations_have_addends = 0
+  } else if (target_arch == "arm64") {
+    relocations_have_addends = 1
+  }
+} else {
+  relocations_have_addends = 0
+  relocation_packer_exe = ""
+}