Use root_out_dir for lib_dirs when building with a custom libc++.

third_party/libc++/BUILD.gn

 # Copyright 2015 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.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Used by libc++ and libc++abi.
 config("config") {
   defines = [ "LIBCXX_BUILDING_LIBCXXABI" ]
@@ skipping 104 matching lines @@
 config("link_helper") {
   ldflags = [
     "-stdlib=libc++",
 
     # Normally the generator takes care of RPATH. Our case is special because
     # the generator is unaware of the libc++.so dependency. Note that setting
     # RPATH here is a potential security issue. See the following for another
     # example of this issue: https://code.google.com/p/gyp/issues/detail?id=315
     "-Wl,-R,\$ORIGIN/",
   ]
-  lib_dirs = [ root_build_dir ]
+  lib_dirs = [ root_out_dir ]
 }