GN: Fix NaCl IRT build options

build/toolchain/nacl_toolchain.gni

 # 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.
 
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 
 # This template defines a NaCl toolchain.
 #
 # It requires the following variables specifying the executables to run:
@@ skipping 25 matching lines @@
                            [
                              "ar",
                              "cc",
                              "cxx",
                              "deps",
                              "is_clang",
                              "is_nacl_glibc",
                              "ld",
                              "link_outputs",
                              "postlink",
+                             "symbol_level",
                              "toolchain_cpu",
                            ])
 
     # We do not suport component builds or sanitizers with the NaCl toolchains.
     is_component_build = false
     clear_sanitizers = true
     use_ccache = false
     use_goma = false
 
     rebuild_define = "NACL_TC_REV=" + invoker.toolchain_revision
   }
 }