link_elf_obj: Handle init_array sections in KLDs

Reuse existing handling for .ctors, print a warning if multiple
constructor sections are present. Destructors are not handled as of
yet.

This is required for KASAN.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29049