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

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

Reviewed By: #libc, abhina.sreeskantharajan, muiez, ldionne

Differential Revision: https://reviews.llvm.org/D111226