Remove platform-specific things from "common" segment files

[ethteck]

Spimdisasm lives in most segment files under segtypes/common/ now. It's not a huge deal, but technically spimdisasm is mips only, so there's nothing "common" about it.

We should probably move the existing "common" files into a mips subdirectory and have n64 and psx files extend the mips versions of all the classes, changing things as needed. Common can contain basic platform-agnostic stuff, or it can just go away if there's deemed to not be enough of that to warrant that abstraction layer