GYP changes for arm64 linux build

build/toolchain/linux/BUILD.gn

 # Copyright 2013 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.
 
 import("//build/config/sysroot.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 
 clang_toolchain("clang_arm") {
   toolchain_cpu = "arm"
   toolchain_os = "linux"
   toolprefix = "arm-linux-gnueabihf-"
 }
 
+clang_toolchain("clang_arm64") {
+  toolchain_cpu = "arm64"
+  toolchain_os = "linux"
+  toolprefix = "aarch64-linux-gnu-"
+}
+
 gcc_toolchain("arm") {
   toolprefix = "arm-linux-gnueabihf-"
 
   cc = "${toolprefix}gcc"
   cxx = "${toolprefix}g++"
 
   ar = "${toolprefix}ar"
   ld = cxx
   readelf = "${toolprefix}readelf"
   nm = "${toolprefix}nm"
@@ skipping 53 matching lines @@
   ld = cxx
   readelf = "mipsel-linux-gnu-readelf"
   nm = "mipsel-linux-gnu-nm"
 
   toolchain_cpu = "mipsel"
   toolchain_os = "linux"
   is_clang = false
   cc_wrapper = ""
   use_goma = false
 }