Add CPU architecture override

I run a znver-4 machine, but due to turning off the iGPU it reports as x86_64-v4. While I can correct for this after an install, it would be a huge QoL improvement to have an option to select the system architecture with it defaulting to an 'auto' mode.

Start by locating the installer’s current CPU-architecture detection and the code that defines user-selectable installation options. Trace how the detected architecture is passed through installation, then determine where an “auto” default and explicit override belong. Done means users can select an architecture, while “auto” preserves detection and supports the reported znver-4 case.

Written by the indexing model from the issue text.