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:		257e70f1d5 kyua: Add FreeBSD Jail execution environment support
Differential Revision:	https://reviews.freebsd.org/D46041

contrib/kyua/engine/atf.cpp

@@ -196,6 +196,7 @@ engine::atf_interface::exec_test(const model::test_program& test_program,
     auto e = execenv::get(test_program, test_case_name);
     e->init();
     e->exec(args);
+    __builtin_unreachable();
 }
 
 
@@ -227,6 +228,7 @@ engine::atf_interface::exec_cleanup(
 
     auto e = execenv::get(test_program, test_case_name);
     e->exec(args);
+    __builtin_unreachable();
 }
 
 

contrib/kyua/engine/plain.cpp

@@ -110,6 +110,7 @@ engine::plain_interface::exec_test(
     auto e = execenv::get(test_program, test_case_name);
     e->init();
     e->exec(args);
+    __builtin_unreachable();
 }
 
 

contrib/kyua/engine/tap.cpp

@@ -157,6 +157,7 @@ engine::tap_interface::exec_test(
     auto e = execenv::get(test_program, test_case_name);
     e->init();
     e->exec(args);
+    __builtin_unreachable();
 }