[libc] Define __UTS_NAME_LENGTH for __APPLE__

[alfredfo]

Summary

Before https://github.com/llvm/llvm-project/commit/cae84d8acf5ec7a8f7b9fd2ce33397bcc22c590c all linux checks were incorrectly unix checks. unix being true on macOS systems therefore meant that macOS would use 65 as __UTS_NAME_LENGTH.

This commit correctly specifices __UTS_NAME_LENGTH to match XNU as 256.
https://opensource.apple.com/source/xnu/xnu-201/bsd/sys/utsname.h.auto.html

[alfredfo]

update commit message

libc/include/llvm-libc-types/struct_utsname.h

 //===-- Definition of struct utsname --------------------------------------===//
 //
 // 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 __LLVM_LIBC_TYPES_STRUCT_UTSNAME_H__
 #define __LLVM_LIBC_TYPES_STRUCT_UTSNAME_H__
 
-#ifdef __linux__
+#if defined(__linux__)
 #define __UTS_NAME_LENGTH 65
+#elif defined(__APPLE__)
+#define __UTS_NAME_LENGTH 256
 #else
 // Arbitray default. Should be specialized for each platform.
 #define __UTS_NAME_LENGTH 1024
 #endif
 
 struct utsname {
   char sysname[__UTS_NAME_LENGTH];
   char nodename[__UTS_NAME_LENGTH];