ar: enable reproducible output by default when invoked as 'ar -s'

ar output is already deterministic by default for ar -q and ar -r, and
when invoked as ranlib. Make ar -s equivalent to ranlib and enable
deterministic output by default in that case too.

PR: 210330
Reviewed by: bdrewery
Approved by: re (gjb)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6871