GN: Build with a custom libc++ when using ASAN, TSAN or MSAN.

build/config/sanitizers/sanitizers.gni

 # 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.
 
 declare_args() {
   # Track where uninitialized memory originates from. From fastest to slowest:
   # 0 - no tracking, 1 - track only the initial allocation site, 2 - track the
   # chain of stores leading from allocation site to use site.
   msan_track_origins = 2
 
   # Use dynamic libraries instrumented by one of the sanitizers instead of the
   # standard system libraries. Set this flag to download prebuilt binaries from
   # GCS.
   use_prebuilt_instrumented_libraries = false
+
+  # Use libc++ (buildtools/third_party/libc++ and
+  # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
+  # This is intended to be used for instrumented builds.
+  use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan
 }