1.0.25.34: gc trigger improvements

1.0.25.34: gc trigger improvements

- Make sure that gc never runs user code (after gc hooks and
  finalizers) if interrupts are disabled and they cannot be enabled
  (i.e. *ALLOW-WITH-INTERRUPTS* is NIL).

- clean up maybe_gc: we know that in the interrupted context gc
  signals are unblocked (see "check that gc signals are unblocked"
  commit) so it's safe to simply enable them and call SUB-GC