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

third_party/libc++abi/src/cxa_handlers.hpp

+//===------------------------- cxa_handlers.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.
+//
+//
+// This file implements the functionality associated with the terminate_handler,
+//   unexpected_handler, and new_handler.
+//===----------------------------------------------------------------------===//
+
+#ifndef _CXA_HANDLERS_H
+#define _CXA_HANDLERS_H
+
+#include <exception>
+
+namespace std
+{
+
+__attribute__((visibility("hidden"), noreturn))
+void
+__unexpected(unexpected_handler func);
+
+__attribute__((visibility("hidden"), noreturn))
+void
+__terminate(terminate_handler func) _NOEXCEPT;
+
+}  // std
+
+extern "C"
+{
+
+extern void (*__cxa_terminate_handler)();
+extern void (*__cxa_unexpected_handler)();
+extern void (*__cxa_new_handler)();
+
+/*
+
+    At some point in the future these three symbols will become
+    C++11 atomic variables:
+
+    extern std::atomic<std::terminate_handler>  __cxa_terminate_handler;
+    extern std::atomic<std::unexpected_handler> __cxa_unexpected_handler;
+    extern std::atomic<std::new_handler>        __cxa_new_handler;
+
+    This change will not impact their ABI.  But it will allow for a
+    portable performance optimization.
+
+*/
+
+} // extern "C"
+
+#endif  // _CXA_HANDLERS_H