[libc++] Fix largefile handling in fs::copy_file (llvm#121855)

Fix for issues reported in llvm#109211

Author: Jannik2099

libcxx/src/filesystem/operations.cpp

@@ -238,8 +238,14 @@ bool copy_file_impl_copy_file_range(FileDescriptor& read_fd, FileDescriptor& wri
     return false;
   }
   // do not modify the fd positions as copy_file_impl_sendfile may be called after a partial copy
+#  if defined(__linux__)
+  loff_t off_in  = 0;
+  loff_t off_out = 0;
+#  else
   off_t off_in  = 0;
   off_t off_out = 0;
+#  endif
+
   do {
     ssize_t res;