kyua: fix gcc builds

For some reason execenv::exec() isn't successfully marked noreturn
(rlibby reports that virtual functions can't be noreturn), but calling
methods are so gcc rightly complains. Work around this by adding
explicit __builtin_unreachable() calls.

Reviewed by: imp, markj
Fixes: 257e70f1d5ee kyua: Add FreeBSD Jail execution environment support
Differential Revision: https://reviews.freebsd.org/D46041

(cherry picked from commit 47fb5d2b13c5ca8e1a3b05ed223d62ea4b43b3e8)

Approved by: kp (mentor), markj (mentor)