workaround for RT #100399

Since perl 5.16.0, global destruction is more eager and is causing to
fire in child processes, usually causing aborts and core dumps. This
is affecting async.t and fork.t, but probably also programs using
fork() within Tk.

Workaround is to use POSIX::_exit instead of CORE::exit, so global
destruction does not fire in child processes.
add issue RT #100399 to ToDo list