Add support for 32-bit and 64-bit Windows compiles in GN.

build/config/win/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.
 
 declare_args() {
   # Full path to the Windows SDK, not including a backslash at the end.
   windows_sdk_path = "C:\Program Files (x86)\Windows Kits\8.0"
 
   # Full path to the Visual Studio installation, not including a backslash
   # at the end.
@@ skipping 26 matching lines @@
     "$windows_sdk_path\Include\shared",
     "$windows_sdk_path\Include\um",
     "$windows_sdk_path\Include\winrt",
     "$visual_studio_path\VC\include",
     "$visual_studio_path\VC\atlmfc\include",
   ]
 }
 
 # Linker flags for Windows SDK setup, this is applied only to EXEs and DLLs.
 config("sdk_link") {
-  # TODO(brettw) 64-bit.
-  is_64bit = false
-
-  if (is_64bit) {
+  if (cpu_arch == "x64") {
     ldflags = [ "/MACHINE:X64" ]
     lib_dirs = [
       "$windows_sdk_path\Lib\win8\um\x64",
       "$visual_studio_path\VC\lib\amd64",
       "$visual_studio_path\VC\atlmfc\lib\amd64",
     ]
   } else {
     ldflags = [
       "/MACHINE:X86",
       "/SAFESEH",  # Not compatible with x64 so use only for x86.
@@ skipping 78 matching lines @@
 }
 
 # Incremental linking ----------------------------------------------------------
 
 config("incremental_linking") {
   ldflags = [ "/INCREMENTAL" ]
 }
 config("no_incremental_linking") {
   ldflags = [ "/INCREMENTAL:NO" ]
 }