elfctl: avoid touching file if no change made

Suggested by: brooks
Reviewed by: brooks, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30452

(cherry picked from commit 3f2508b7f3855102abed99b846e30e728ba3d04d)
(cherry picked from commit 5ceb90aa667afbd0941f045073d54aea35d40f1b)