set assembler section header fields

[katsyoshi]

Summary

• compute section header fields from section type/name in assembler writer
• set flags/addralign/info/entsize consistently for generated ELF sections
• align compiler test to use self linker for tmp output
• add section header flag constants for future use

Testing

• rake test

[chatgpt-codex-connector]

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[katsyoshi]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: cd8446b503

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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