rust: add support for armv6l-linux and armv7l-linux

[lopsided98]

Motivation for this change

Extremely similar to #72480, except with a different mapping function name and location, bootstrap tarball hashes for armv6l, and another platform string fixup in .cargo/config.

Note that the rust does not build on armv6l right now because bootstrap rustc segfaults while building libc since the upgrade (of the bootstrap compiler) to 1.38.0.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

[lopsided98]

1.39.0 does not build on armv6l or armv7l due to rust-lang/rust#62896. I was able to successfully build 1.38.0 for armv6l.