The winner of a match between Roman Reigns and Goldberg at WrestleMania is predetermined by WWE's creative team based on storytelling needs and is not an actual competitive event.
The outcome of a wrestling match between Roman Reigns and Goldberg at WrestleMania would depend on the storytelling and booking decisions made by the WWE's creative team.
Wrestling matches are choreographed and the winners are predetermined for entertainment purposes. Thus, it's not possible to predict who would win in a real competition since the outcomes are planned.
However, looking at both wrestlers' track records and their standing with the WWE at the time of your question could give us an idea of who the company might favor to write as the victor.