dirent-util: introduce simple wrapper of posix_getdents()

glibc exports getdents64 syscall as is, but musl exports it as
posix_getdents(). Let's introduce a simple wrapper of posix_getdents().

Note, our baseline for glibc is 2.31. Hence, we can assume getdents64()
always defined when building with glibc.

src/basic/missing_syscall.h

@@ -212,16 +212,6 @@ static inline int missing_close_range(unsigned first_fd, unsigned end_fd, unsign
 
 /* ======================================================================= */
 
-#if !HAVE_GETDENTS64
-static inline ssize_t missing_getdents64(int fd, void *buffer, size_t length) {
-        return syscall(__NR_getdents64, fd, buffer, length);
-}
-
-#  define getdents64 missing_getdents64
-#endif
-
-/* ======================================================================= */
-
 #if !HAVE_SCHED_SETATTR
 /* since kernel 3.14 (e6cfc0295c7d51b008999a8b13a44fb43f8685ea) */
 static inline ssize_t missing_sched_setattr(pid_t pid, struct sched_attr *attr, unsigned int flags) {