Do not erronously export 'openat' symbol from rtld.

The symbol leaked after r276630 since lib/libc/sys/openat.c defines
versions for openat using .symver (version script cannot assign two
versions to one symbol), and rtld uses openat. Instead, directly use
__sys_openat().

Reported and tested by: antoine
Sponsored by: The FreeBSD Foundation
MFC after: 1 week