libsys auxv.c: add fences needed to ensure that flag works

Noted by: jrtc27
Sponsored by: The FreeBSD Foundation

libsys: remove usage of pthread_once and _once_stub

that existed in auxv.c, use simple bool gate instead. This leaves a
small window if two threads try to call _elf_aux_info(3) simultaneously.
The situation is safe because auxv parsing is really idempotent. The
parsed data is the same, and we store atomic types (int/long/ptr) so
double-init does not matter.

Reviewed by: brooks, imp
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D43985

libc: elf auxiliary vector handling to libsys

This is part of the interface to the kernel and some syscall wrappers
depend on it so move it there.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908