Add libc++ and libc++abi to third-party.

third_party/libc++abi/src/exception.cpp

+//===---------------------------- exception.cpp ---------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <exception>
+
+#pragma GCC visibility push(default)
+
+namespace std
+{
+
+// exception
+
+exception::~exception() _NOEXCEPT
+{
+}
+
+const char* exception::what() const _NOEXCEPT
+{
+  return "std::exception";
+}
+
+// bad_exception
+
+bad_exception::~bad_exception() _NOEXCEPT
+{
+}
+
+const char* bad_exception::what() const _NOEXCEPT
+{
+  return "std::bad_exception";
+}
+
+}  // std
+
+#pragma GCC visibility pop