How does DO-254 apply to commercial processors? For instance, I've noticed that "DO-254 certifiable" processors exist. Does that mean that these were designed with inherent redundancy?
DO-254 is for Airborne Electronic Hardware. Redundancy isn't necessarily required for DO-254. I would expect that the processor manufacturer would be able to provide a data package with the necessary documentation if a company were to use it in a DO-254 certifiable product.
In my experience, however, DO-254 is applied for custom micro-coded components only (for the FAA at least). DO-254 was a way to apply concepts similar to DO-178 to things like PLDs, FPGAs, and ASICs to prevent a "firmware" gap in certification.