Fix libc++ ARM build

third_party/libunwind/BUILD.gn

+# Copyright 2017 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.
+
+config("libunwind_config") {
+  # ValueAsBitPattern in Unwind-EHABI.cpp is only used on Debug builds.
+  cflags = [ "-Wno-unused-function" ]
+}
+
+source_set("libunwind") {
+  visibility = [ "//buildtools/third_party/libc++abi" ]
+  sources = [
+    # C++ sources
+    "trunk/src/Unwind-EHABI.cpp",
+    "trunk/src/libunwind.cpp",
+
+    # C sources
+    "trunk/src/Unwind-sjlj.c",
+    "trunk/src/UnwindLevel1-gcc-ext.c",
+    "trunk/src/UnwindLevel1.c",
+
+    # ASM sources
+    "trunk/src/UnwindRegistersRestore.S",
+    "trunk/src/UnwindRegistersSave.S",
+  ]
+  configs -= [
+    "//build/config/compiler:chromium_code",
+    "//build/config/compiler:no_rtti",
+    "//build/config/gcc:no_exceptions",
+  ]
+  configs += [
+    "//build/config/compiler:no_chromium_code",
+    "//build/config/compiler:rtti",
+    "//buildtools/third_party/libc++:config",
+
+    # Must be after no_chromium_code
+    ":libunwind_config",
+  ]
+}