The 64-bit libc on Solaris and illumos only has readdir_r. If a
32-bit Solaris or illumos target is ever created, it should use
__posix_readdir_r. See libc(3LIB) on Solaris or illumos:
https://illumos.org/man/3lib/libc
https://docs.oracle.com/cd/E36784_01/html/E36873/libc-3lib.html
https://www.unix.com/man-page/opensolaris/3LIB/libc/