1.0.25.37: block deferrables when gc pending in PA

1.0.25.37: block deferrables when gc pending in PA

Consider this:

  set_pseudo_atomic_atomic()
  alloc and set pseudo_atomic_interrupted and GC_PENDING
  clear_pseudo_atomic_atomic()
  if (get_pseudo_atomic_interrupted())
    gc();

If an async interrupt happens and unwinds between
clear_pseudo_atomic_atomic and gc() we have lost a gc trigger until
the next alloc. Same with SIG_STOP_FOR_GC instead of alloc.

This patch addresses the above problem by blocking deferrables when a
gc is requested in a pseudo atomic section. On exit from the protected
section there is no race because no async unwinding interrupts can
happen.