Actually, what computer architecture would the Cybermen even have? I like to imagine that the parallel-Earth Cybermen just have a regular-ass PC motherboard somewhere inside them (they were manufactured on present-day Earth, after all), and they've simply never bothered to change that part of their design because too much of their codebase relies on quirks of the x86 processor and the IBM PC architecture. Maybe they're still trying to track down and upgrade the One Guy™ who wrote all the crazy ASM hacks for Cybus, then fucked off to Argentina to become a farmer.

By contrast, given the "bodged-together" nature of the Mondasian Cybermen (they were created on a colony ship experiencing time dilation as a frantic attempt to prolong the crew's lives), they likely started off with all sorts of varying CPU architectures - simply whatever components they had access to. Maybe they run some sort of Mondasian Java... now there's a horrifying thought for you!