Do not pass default values from GN to ld_bfd.py if CXX is specified

src/trusted/service_runtime/linux/BUILD.gn

 # Copyright (c) 2014 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # We use a special toolchain to build nacl_helper_bootstrap.  This
 # seems a bit excessive for compiling one source file.  But it is
 # the most reasonable way to instantiate the configs with different
 # settings such as use_gold=false.  The minimal set of configs used
 # for that file (see below) will sometimes try to use the switch
 # -gsplit-dwarf under use_gold=true.  But without some other configs
@@ skipping 32 matching lines @@
   }
 
   if (is_android) {
     compiler = "g++"
   } else if (current_cpu == "arm") {
     compiler = "arm-linux-gnueabihf-g++"
   } else {
     compiler = "g++"
   }
 
+  if (target_os == "chromeos" && defined(cros_target_cxx) &&
+      cros_target_cxx != "") {
+    compiler = cros_target_cxx
+  }
+
   static_library("nacl_bootstrap_lib") {
     # This code has very specific requirements related to its static
     # linking and minimal libc use.  It cannot support the normal variety
     # of build configs that the rest of the code is built with.  So it
     # resets the list of configsto empty, and then builds up from scratch
     # using only the basics that work here.
     configs = []
     configs += [
       "//build/config/compiler:compiler_cpu_abi",
       "//build/config/compiler:runtime_library",
@@ skipping 92 matching lines @@
   copy("bootstrap") {
     bootstrap = ":munge_nacl_helper_bootstrap($bootstrap_toolchain)"
 
     bootstrap_out_dir = get_label_info(bootstrap, "root_out_dir")
 
     deps = [ bootstrap ]
     sources = [ "$bootstrap_out_dir/nacl_helper_bootstrap" ]
     outputs = [ "$root_out_dir/nacl_helper_bootstrap" ]
   }
 }