doc/hacking.md: Corrections and additions for cross

[roberth]

Motivation

• "Target" has a specific meaning in cross-compilation, which does not apply here, and we can avoid the term without loss.
• Cross compilation wasn't mentioned yet. This isn't a guide to bootstrapping Nix on a platform, but we can't not mention it here, as otherwise the section suggests that emulation is the only way to port Nix.

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

[roberth]

Thanks!

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-45/26397/1