Add compiler and linker flags for MIPS.

build/config/mips.gni

+# Copyright 2015 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.
+
+# MIPS arch variant.
+if (cpu_arch == "mipsel") {
+  declare_args() {
+    mips_arch_variant = "r1"
+  }
+} else if (cpu_arch == "mips64el") {
+  if (is_android) {
+    declare_args() {
+      mips_arch_variant = "r6"
+    }
+  } else {
+    declare_args() {
+      mips_arch_variant = "r2"
+    }
+  }
+}