> Can the question be migrated over to StackOverflow?
No. It's far too old. And off-topic there as well.
> Can I bounty one of his other answers on an open question, and state in the bounty notice that it's for the x86 answer?
Technically, maybe. But I would consider this an abuse of the system. Votes and bounties should be bound exclusively to the specific question or answer, not on some random other stuff to game the system.
Really, the answer has 136 upvotes. For Serverfault, this is excellent and there is no need at all to do anything more except giving an upvote. And quite frankly, this is all more or less unimportant trivia anyway.