tzsetup: add support for countries in two continents

This supports countries located across multiple continents, as per the
zone1970.tab file. This only affects Cyprus and Türkiye at the moment.

PR: 236874
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41306

(cherry picked from commit 914ab28c598c108bcfa5c040cc3056a8e6fc3811)