Rework handling of os and cpu_arch in GN.

tools/gn/misc/vim/syntax/gn.vim

 " Copyright 2014 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.
 "
 " gn.vim: Vim syntax file for GN.
 "
 " Quit when a (custom) syntax file was already loaded
 "if exists("b:current_syntax")
   "finish
 "endif
 
 syn case match
 
 " Keywords within functions
 syn keyword     gnConditional       if else
 hi def link     gnConditional       Conditional
 
 " Predefined variables
-syn keyword     gnPredefVar build_cpu_arch build_os cpu_arch current_toolchain
-syn keyword     gnPredefVar default_toolchain os python_path root_build_dir
-syn keyword     gnPredefVar root_gen_dir root_out_dir target_gen_dir
-syn keyword     gnPredefVar target_out_dir
+syn keyword     gnPredefVar current_cpu current_os current_toolchain
+syn keyword     gnPredefVar default_toolchain host_cpu host_os
+syn keyword     gnPredefVar root_build_dir root_gen_dir root_out_dir
+syn keyword     gnPredefVar target_cpu target_gen_dir target_out_dir
+syn keyword     gnPredefVar target_os
 syn keyword     gnPredefVar true false
 hi def link     gnPredefVar         Constant
 
 " Target declarations
 syn keyword     gnTarget action action_foreach copy executable group
 syn keyword     gnTarget shared_library source_set static_library
 hi def link     gnTarget            Type
 
 " Buildfile functions
 syn keyword     gnFunctions assert config declare_args defined exec_script
@@ skipping 38 matching lines @@
 "syn match gnOperator /-=/
 "syn match gnOperator /\s>\s/
 "syn match gnOperator /\s<\s/
 "syn match gnOperator /\s+\s/
 "syn match gnOperator /\s-\s/
 "hi def link     gnOperator          Operator
 
 syn sync minlines=500
 
 let b:current_syntax = "gn"