Fail HOL-Light tests on unsupported arch

[fegge]

Summary

• Make the HOL-Light test command fail on unsupported host architectures instead of reporting success.
• Keep supported architecture mapping explicit before selecting proof functions.

Validation

• ruff format scripts/tests: passed
• ruff check scripts/tests: passed
• python3 -m py_compile scripts/tests: passed
• git diff --check -- scripts/tests: passed
• python3 scripts/tests hol_light --list-functions: passed
• Unsupported-architecture probe with platform.machine() mocked to riscv64: failed as expected with HOL-Light unsupported architecture: riscv64
• Not run: ./scripts/format because nixpkgs-fmt is not installed in this local environment
• Not run: ./scripts/lint because shfmt is not installed in this local environment

Fixes #1214

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This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The issue was identified primarily by the Codex coding agent, and manually reviewed before submission.

[fegge]

Reopened from an upstream repository branch with DCO-signed commits so full CI can run: #1232