1.0.25.23: more allocation checks

1.0.25.23: more allocation checks

- in gencgc alloc() check that we are in pseudo atomic
- assert that interrupt_handle_pending does not happen in pseudo atomic
- assert that cheneygc pa_alloc() runs with deferrable signals blocked
- add test code to trigger gc from pa_alloc to help testing how the
  runtime deals with such rare events