Deduplicate codegen backends in bootstrap config

[kn1g78]

Deduplicate rust.codegen-backends and target.<triple>.codegen-backends during bootstrap config parsing while preserving first occurrence order.

This avoids carrying redundant backend entries into later bootstrap planning.

test:

$env:CARGO_NET_OFFLINE='false'; $env:CARGO_TARGET_DIR='target-codex-test'; $cargo = Join-Path $env:USERPROFILE '.cargo\bin\cargo.exe'; & $cargo test --manifest-path src/bootstrap/Cargo.toml deduplicates_codegen_backends

result:

running 1 test
test core::config::tests::deduplicates_codegen_backends ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 255 filtered out; finished in 0.00s

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @jieyouxu (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789, jieyouxu