Add SSE2/MMX build flags when building on an x86 host. This is necessary

build/config/compiler/BUILD.gn

 # Copyright (c) 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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 466 matching lines @@
     # simplicity we always explicitly set the architecture.
     if (current_cpu == "x64") {
       cflags += [
         "-m64",
         "-march=x86-64",
       ]
       ldflags += [ "-m64" ]
     } else if (current_cpu == "x86") {
       cflags += [ "-m32" ]
       ldflags += [ "-m32" ]
+      if (!is_nacl) {
+        cflags += [
+          "-msse2",
+          "-mfpmath=sse",
+          "-mmmx",
+        ]
+      }
     } else if (current_cpu == "arm") {
       if (is_clang && !is_android && !is_nacl) {
         cflags += [
           "-target",
           "arm-linux-gnueabihf",
         ]
         ldflags += [
           "-target",
           "arm-linux-gnueabihf",
         ]
@@ skipping 952 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }