ci: eval arm qemu on fleets

[dougch]

Issues:

Resolves #ISSUE-NUMBER1
Addresses #ISSUE-NUMBER2

Description of changes:

Describe AWS-LC’s current behavior and how your code changes that behavior. If there are no issues this pr is resolving, explain why this change is necessary.

Call-outs:

Point out areas that need special attention or support during the review process. Discuss architecture or design changes.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.14%. Comparing base (9651480) to head (eb5d30f).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3241      +/-   ##
==========================================
+ Coverage   78.12%   78.14%   +0.02%     
==========================================
  Files         689      689              
  Lines      123214   123263      +49     
  Branches    17137    17139       +2     
==========================================
+ Hits        96257    96322      +65     
+ Misses      26047    26032      -15     
+ Partials      910      909       -1     

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[dougch]

Observations:

• CodeBuild GHA runners have no managed Ubuntu ARM image (only AL2/AL2023)
• AL2023 has no qemu packages
• GitHub-hosted runners don't expose KVM, so cross-platform-actions/action needs QEMU userspace emulation to run the OpenBSD VM for ARM targets
• On x86 ubuntu-latest this "just works" because Ubuntu ships QEMU packages