elfcopy: Optimize for insertions at the end of the section list.

This is the common case when strip(1) is creating the output file.
The change provides a significant speedup when running on ELF files with
many sections.

PR: 234949
Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20444