AArch64: Add 128-bit integer types definitions

This patch adds 128-bit integer types int128_t and uint128_t
for "__int128" and "unsigned __int128" supported by GCC and
Clang for AArch64.

Change-Id: I0e646d026a5c12a09fd2c71dc502082052256a94
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>

include/lib/libc/aarch64/stdint_.h

 /*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2018-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -116,3 +116,6 @@ typedef unsigned long uintmax_t;
 
 typedef long register_t;
 typedef unsigned long u_register_t;
+
+typedef __int128 int128_t;
+typedef unsigned __int128 uint128_t;