stdenv: disable hardening for embedded (kernel=none) targets

[midnightveil]

A very partial fix for #18995. This should make the relatively common case of embedded targets Just Work™, since these systems rarely are setup in the standard way.

I've often seen __stack__chk failures when building for embedded systems, enabling this should make these architectures usable without needing to do crimes in nix shells / build environments.

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DRAFT, because I need to make sure this works as expected, i.e. I can build things without adding hardeningDisable to nix-shells for embedded work.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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cc @wucke13 since this might be of shared interest

[wucke13]

@midnightveil Thank you! I do this all the time for embedded targets, this is a net positive in usability of Nix for niche target environments

[midnightveil]

@midnightveil Thank you! I do this all the time for embedded targets, this is a net positive in usability of Nix for niche target environments

Yep, I know :)

I have a suspicion that it might need to be targetPlatform instead of hostPlatform (or that it might not work since the mkShell is usually a non-cross stdenv and might override the compiler)... unfortunately it rebuilds the universe since it modifies most of the target's packages which will take a while.