[libcxx] Mark a test as unsupported for C++03

[miyuki]

Summary

The nullptr_t_integral_cast.pass.cpp test is currently xfailed for
C++03, but actually, it only fails with the first version of libc++
ABI.

This patch changes XFAIL to UNSUPPORTED to avoid unexpected passes
with ABI v2 or later.

libcxx/test/std/language.support/support.types/nullptr_t_integral_cast.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-// NOTE: nullptr_t emulation cannot handle a reinterpret_cast to an
-// integral type
-// XFAIL: c++03
+// NOTE: nullptr_t emulation (used in libc++ ABI v.1) cannot handle a
+// reinterpret_cast to an integral type
+// UNSUPPORTED: c++03
 
 // typedef decltype(nullptr) nullptr_t;
 
 
 #include <cstddef>
 #include <cassert>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     std::ptrdiff_t i = reinterpret_cast<std::ptrdiff_t>(nullptr);
     assert(i == 0);
 
   return 0;
 }