Support for arm64 build.

build/config/arm.gni

Index: build/config/arm.gni
diff --git a/build/config/arm.gni b/build/config/arm.gni
index 6d63965e39ddf4c0d9f3b58ec4396b25a32cca4d..8cbbf9a6e8b3fb659537b5da5b230a3902f66e9d 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -2,11 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-if (current_cpu == "arm") {
+if (current_cpu == "arm" || current_cpu == "arm64") {
   declare_args() {
     # Version of the ARM processor when compiling on ARM. Ignored on non-ARM
     # platforms.
-    arm_version = 7
+    if (current_cpu == "arm64") {
+      arm_version = 8
+    } else {
+      arm_version = 7
+    }
 
     # The ARM floating point mode. This is either the string "hard", "soft", or
     # "softfp". An empty string means to use the default one for the