In a GM, or other, bankruptcy the priority of government "loans", "preferred stock" et. al. depends partly on what sort of documents were signed by company officers when they took the money, and partly on what subordination agreements (from bond holders or lenders) were not sought at the time.
I assume it was a similar deal as with the banks where the priority is clear, or some attorney at Treasury didn't do their job right.
I doubt there was any new law tucked into the stimulus bill giving government money superpriority in bankruptcy. And even if there were, it wouldn't be binding on existing bond holders or lenders. In general, bankruptcy superpriority is only granted to debtor-in-posession lendors.
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