Add repro-check tool for verifying reproducible Rust builds

[windsunil]

This patch introduces a new host tool called repro-check that helps verify whether Rust compiler builds are reproducible.

Summary:
-> Builds the stage-2 Rust toolchain (or a full distribution) twice in separate workspaces.
-> Compares the resulting sysroots byte-for-byte using SHA-256.
-> Generates a detailed HTML report highlighting mismatches and ignored files.
-> Writes a deterministic bootstrap.toml for each build to ensure consistent settings.
-> Supports parallel hashing via Rayon.
-> Accepts standard options such as:
--jobs – Number of parallel jobs
--exclude-pattern – Ignore files matching a pattern
--path-delta – Adds extra directory levels to second build for path-sensitivity testing
--full-dist – Build full distribution instead of stage 2
--clean – Start from a clean workspace
--skip-copy – Reuse existing workspace without copying source
--verbose – Print detailed logging

Included components:
-> src/tools/repro-check/ – Tool source code (main.rs, build.rs, compare.rs, fs_utils.rs, config.rs)
-> Integration tests for hash computation, mismatch detection, ignored patterns, and case-insensitive handling.
-> Updates to Cargo.toml and Cargo.lock to include repro-check dependencies.
-> Bootstrap and builder updates to register the new tool.

References:
Tracking issues: #139793, #134589, #144669

Usage:

Build the tool

./x.py build src/tools/repro-check

Run (stage2 only, host target)

./build/x86_64-unknown-linux-gnu/stage1-tools-bin/repro-check

Full distribution with custom options

./build/x86_64-unknown-linux-gnu/stage1-tools-bin/repro-check
--jobs 16
--exclude-pattern .so
--path-delta 10
--html-output my-report.html

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rustbot]

r? @Mark-Simulacrum

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