Fix //tools/gn/example on Linux: missing current_os and related vars

tools/gn/example/build/BUILDCONFIG.gn

 # 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.
 
+if (target_os == "") {
+  target_os = host_os
+}
+if (target_cpu == "") {
+  target_cpu = host_cpu
+}
+if (current_cpu == "") {
+  current_cpu = target_cpu
+}
+if (current_os == "") {
+  current_os = target_os
+}
+
 # All binary targets will get this list of configs by default.
 _shared_binary_target_configs = [ "//build:compiler_defaults" ]
 
 # Apply that default list to the binary target types.
 set_defaults("executable") {
   configs = _shared_binary_target_configs
 
   # Executables get this additional configuration.
   configs += [ "//build:executable_ldconfig" ]
 }
 set_defaults("static_library") {
   configs = _shared_binary_target_configs
 }
 set_defaults("shared_library") {
   configs = _shared_binary_target_configs
 }
 set_defaults("source_set") {
   configs = _shared_binary_target_configs
 }
 
 set_default_toolchain("//build/toolchain:gcc")