csu: C standard requires errno on main() entry

Reported by: Greg Becker <greg.becker@klarasystems.com>
Reviewed by: emaste, kevans, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Also see: https://reviews.freebsd.org/D50998
Differential revision: https://reviews.freebsd.org/D50997