libsys: don't expose __init_elf_aux_vector

__init_elf_aux_vector is now complied statically so don't try to export
it from the dynamic library.

Fixes: 8271d9b99a3b libsys: remove usage of pthread_once and _once_stub

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44237