Always set symbol visibility when compiling with GCC / Clang.

third_party/libc++abi/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.
 
 config("libc++abi_warnings") {
   if (is_clang) {
     # http://llvm.org/PR25978
     cflags = [ "-Wno-unused-function" ]
   }
 }
@@ skipping 21 matching lines @@
   ]
   configs -= [
     "//build/config/compiler:chromium_code",
     "//build/config/compiler:no_rtti",
     "//build/config/gcc:no_exceptions",
     "//build/config/gcc:symbol_visibility_hidden",
   ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//build/config/compiler:rtti",
+    "//build/config/gcc:symbol_visibility_default",
     "//buildtools/third_party/libc++:config",
     # Must be after no_chromium_code.
     ":libc++abi_warnings",
   ]
 }