Rework handling of os and cpu_arch in GN.

tools/gn/command_args.cc

 // 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.
 
 #include <stdio.h>
 #include <stdlib.h>
 
 #include <map>
 
 #include "base/command_line.h"
@@ skipping 304 matching lines @@
     "        --args=<exact list of args to use>\n"
     "\n"
     "Examples\n"
     "  gn args out/Debug\n"
     "    Opens an editor with the args for out/Debug.\n"
     "\n"
     "  gn args out/Debug --list --short\n"
     "    Prints all arguments with their default values for the out/Debug\n"
     "    build.\n"
     "\n"
-    "  gn args out/Debug --list=cpu_arch\n"
-    "    Prints information about the \"cpu_arch\" argument for the out/Debug\n"
+    "  gn args out/Debug --list=target_cpu_arch\n"
+    "    Prints information about the \"target_cpu_arch\" argument for the "
+        "out/Debug\n"
     "    build.\n"
     "\n"
     "  gn args --list --args=\"os=\\\"android\\\" enable_doom_melon=true\"\n"
     "    Prints all arguments with the default values for a build with the\n"
     "    given arguments set (which may affect the values of other\n"
     "    arguments).\n";
 
 int RunArgs(const std::vector<std::string>& args) {
   if (args.size() != 1) {
     Err(Location(), "Exactly one build dir needed.",
         "Usage: \"gn args <out_dir>\"\n"
         "Or see \"gn help args\" for more variants.").PrintToStdout();
     return 1;
   }
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchList))
     return ListArgs(args[0]);
   return EditArgsFile(args[0]);
 }
 
 }  // namespace commands