Fix errno enum

Some parts of `Errno` are arch-specific, so `Errno` is currently broken for architectures besides `x86-64` and `aarch64`. Instead, we should have a per-architecture definition of `Errno`.

Start by locating the Errno definition and the architecture-specific code in the repository. Review how errno values differ across supported Linux architectures, then ensure Errno has a definition for each architecture and verify that the existing project checks pass.

Written by the indexing model from the issue text.