Rework handling of os and cpu_arch in GN.

tools/gn/variables.h

 // 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.
 
 #ifndef TOOLS_GN_VARIABLES_H_
 #define TOOLS_GN_VARIABLES_H_
 
 #include <map>
 
 #include "base/strings/string_piece.h"
@@ skipping 35 matching lines @@
 extern const char kRootBuildDir_Help[];
 
 extern const char kRootGenDir[];
 extern const char kRootGenDir_HelpShort[];
 extern const char kRootGenDir_Help[];
 
 extern const char kRootOutDir[];
 extern const char kRootOutDir_HelpShort[];
 extern const char kRootOutDir_Help[];
 
+extern const char kTargetCpuArch[];
+extern const char kTargetCpuArch_HelpShort[];
+extern const char kTargetCpuArch_Help[];
+
+extern const char kTargetOs[];
+extern const char kTargetOs_HelpShort[];
+extern const char kTargetOs_Help[];
+
 extern const char kTargetGenDir[];
 extern const char kTargetGenDir_HelpShort[];
 extern const char kTargetGenDir_Help[];
 
 extern const char kTargetOutDir[];
 extern const char kTargetOutDir_HelpShort[];
 extern const char kTargetOutDir_Help[];
 
 // Target vars -----------------------------------------------------------------
 
@@ skipping 142 matching lines @@
 // Note: this is used only for help so this getter is not threadsafe.
 const VariableInfoMap& GetBuiltinVariables();
 
 // Returns the variables used by target generators.
 // Note: this is used only for help so this getter is not threadsafe.
 const VariableInfoMap& GetTargetVariables();
 
 }  // namespace variables
 
 #endif  // TOOLS_GN_VARIABLES_H_