rustc: use more complete meta.platforms list when doing cross compilation

[SuperSandro2000]

Before building rust-hypervisor-firmware failed like:

 ➜ nix eval -f. rust-hypervisor-firmware
...
...
       error: Package ‘rust-hypervisor-firmware-0.4.2’ in nixpkgs/pkgs/applications/virtualization/rust-hypervisor-firmware/default.nix:60 is not available on the requested hostPlatform:
         hostPlatform.config = "x86_64-elf"
         package.meta.platforms = [ ]
         package.meta.badPlatforms = [
           {
             abi = {
               abi = "n32";
             };
             cpu = {
               bits = 64;
               family = "mips";
             };
           }
         ]
       , refusing to evaluate.

       a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
          for a single invocation of the nix tools.

            $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

          Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                then pass `--impure` in order to allow use of environment variables.

       b) For `nixos-rebuild` you can set
         { nixpkgs.config.allowUnsupportedSystem = true; }
       in configuration.nix to override this.

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnsupportedSystem = true; }
       to ~/.config/nixpkgs/config.nix.

with this change it can be compiled properly:

 ➜ nix eval -f. rust-hypervisor-firmware
/nix/store/b0hlzbvj8wyrpxavb1jxww6c4v674wsa-rust-hypervisor-firmware-x86_64-elf-0.4.2.drv

Motivated by #377247

cc @astro

Reviewer note: to successful test this you must cherry-pick #378694

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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[alyssais]

No, this would be incorrect. This describes which platforms we can build the compiler for. If you made this change, it would express that rustc can be built for any platform that rustc can target, which is not true.

One change that would be correct, though, would be that we could use a broader list for fastCross — in that case , the "rustc" package is actually only libstd, so we could list every platform libstd supports in the platform list.