objcopy: add new sections also when there is no .shstrtab

Previously objcopy (elfcopy) --add-sections inserted new sections before
.shstrtab, but omitted them if there was no .shstrtab.

Now, after processing existing sections add new sections if they were
not yet added.

PR: 241437
Reported by: arrowd
Submitted by: Tiger Gao <tig@FreeBSDFoundation.org>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23571