Nobody really knows at this point. A year or two ago, analysts were saying something about a springing lien by the PBGC if the stock fell below $9, and the PBGC would obtain certain properties as collateral as a mechanism to ensure the integrity of the pension obligations.
I don't think that has ever happened, and obviously, the stock is well below $9 and has been for some time, though it is possible that the sale of Craftsman and the resulting payout into the fund may have staved off the springing lien, but I may be wrong.
Basically, the stock price has little to do with bankruptcy. It is whenever the company is in substantial arrears with creditors, cannot obtain lines credit or when suppliers cut off shipments and/or make demands for payment for the goods in advance. Those factors do have an effect on the price per share, but a decreasing share price does not automatically mean bankruptcy.