Ensure libunwind symbols are hidden

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" ]
+  defines = [ "_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS" ]
   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",
   ]
 }