Correctly set -stdlib=libc++ when building with a custom libc++.

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 38 matching lines @@
     if (defined(invoker.is_clang)) {
       is_clang = invoker.is_clang
     }
     if (defined(invoker.is_nacl_glibc)) {
       is_nacl_glibc = invoker.is_nacl_glibc
     }
     if (defined(invoker.symbol_level)) {
       symbol_level = invoker.symbol_level
     }
 
-    # We do not support component builds or sanitizers with the NaCl toolchains.
+    # We do not support component builds with the NaCl toolchains.
     is_component_build = false
-    clear_sanitizers = true
 
     # We do not support tcmalloc in the NaCl toolchains.
     use_allocator = "none"
 
     rebuild_define = "NACL_TC_REV=" + invoker.toolchain_revision
   }
 }