dirent-util: use getdents64() as is

dirent-util: use getdents64() as is

This partially reverts e86a492ff08526e5adf85fa881c76f80adc3c84a.

The function getdents64() was introduced in glibc-2.30, and our baseline
on glibc is 2.31. Hence, we can assume the function always exists.
The posix_getdents() wrapper was introduced for compatibility with musl.
However, even the latest release of musl does not provide posix_getdents()
yet. Also, even with musl, by defining _LARGEFILE64_SOURCE, we can get
getdents64() and struct dirent64. Hence, the wrapper is anyway not
necessary.