rtld-elf: Mark LD_SHOW_AUXV insecure

This prevents dumping the memory layout of setugid processes.

MFC after: 3 days
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54033

(cherry picked from commit 5242bcff202fa2a5a39895423c8d8c11c02ad76a)