`_readdir64_r_` is deprecated since glibc 2.24

When Compile it on Ubuntu 18.04, there is a warning in `sgx_ustdc/file.c`

```
file.c: In function _u_readdir64_r_ocall_:
file.c:276:5: warning: _readdir64_r_ is deprecated [-Wdeprecated-declarations]
return readdir64_r(dirp, entry, result);
^~~~~~
In file included from file.c:29:0:
/usr/include/dirent.h:201:12: note: declared here
extern int readdir64_r (DIR *__restrict __dirp,
^~~~~~~~~~~
```

Since glibc 2.24, `_readdir64_r_ ` is deprecated. (source : http://man7.org/linux/man-pages/man3/readdir_r.3.html)

Start in sgx_ustdc/file.c at _u_readdir64_r_ocall_ and inspect the readdir64_r call that produces the Ubuntu 18.04 warning. Check the glibc 2.24 deprecation guidance linked in the issue; done means compiling without this deprecation warning while preserving directory-reading behavior.

Written by the indexing model from the issue text.