poly1305 ARM: Clarify layout of state.

[briansmith]

Also, avoid wasting space.

See BoringSSL df41163cb2463d62c9b217ea06aa667a1a01b706.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (1ffab73) to head (468d5c2).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2839      +/-   ##
==========================================
+ Coverage   96.51%   96.52%   +0.01%     
==========================================
  Files         206      206              
  Lines       20530    20526       -4     
  Branches      518      518              
==========================================
- Hits        19814    19813       -1     
+ Misses        599      596       -3     
  Partials      117      117              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.