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

third_party/libc++abi/src/abort_message.h

+//===-------------------------- abort_message.h-----------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef __ABORT_MESSAGE_H_
+#define __ABORT_MESSAGE_H_
+
+#include <stdio.h>
+
+#pragma GCC visibility push(hidden)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+__attribute__((visibility("hidden"), noreturn))
+       void abort_message(const char* format, ...) 
+            __attribute__((format(printf, 1, 2)));
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#pragma GCC visibility pop
+
+#endif
+