[SystemZ][z/OS][libcxx]:Fix fopen64 undeclared error in 32 bit mode

[fanbo-meng]

Summary

z/OS doesn't support fopen64() functions. Modify the preprocessor directive for z/OS to use fopen() instead.

[fanbo-meng]

ping

[abhina.sreeskantharajan]

LGTM, but I think you need a group reviewer approval

[fanbo-meng]

ping for a libc++ group reviewer, thanks.

[fanbo-meng]

@ldionne Could you review this patch please, thanks!

[muiez]

LGTM

libcxx/test/support/filesystem_test_helper.h

#endif
 
     // Purposefully using a size potentially larger than off_t here so we can
     // test the behavior of libc++fs when it is built with _FILE_OFFSET_BITS=64
     // but the caller is not (std::filesystem also uses uintmax_t rather than
     // off_t). On a 32-bit system this allows us to create a file larger than
     // 2GB.
     std::string create_file(fs::path filename_path, uintmax_t size = 0) {
         std::string filename = filename_path.string();
-#if defined(__LP64__) || defined(_WIN32)
+#if defined(__LP64__) || defined(_WIN32) || defined(__MVS__)
         auto large_file_fopen = fopen;
         auto large_file_ftruncate = utils::ftruncate;
         using large_file_offset_t = off_t;
 #else
         auto large_file_fopen = fopen64;
         auto large_file_ftruncate = ftruncate64;
         using large_file_offset_t = off64_t;
 #endif