[libc++][NFC] Add MVS guard for locale_mgmt_zos.h

[daltenty]

Summary

This header need not be included on non-z/OS IBM platforms (and indeed will add nothing when it is), so add a guard. This let's
us remove the header without things breaking when shipping libc++ for AIX.

[fanbo-meng]

LGTM

[hubert.reinterpretcast]

LGTM; thanks!

libcxx/include/__support/ibm/xlocale.h

 // -*- C++ -*-
 //===-----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_SUPPORT_IBM_XLOCALE_H
 #define _LIBCPP_SUPPORT_IBM_XLOCALE_H
 
+#if defined(__MVS__)
 #include <__support/ibm/locale_mgmt_zos.h>
+#endif // defined(__MVS__)
+
 #include <stdarg.h>
 
 #include "cstdlib"
 
 #ifdef __cplusplus
 extern "C" {
 #endif