NFC: CPU: Clarify safety invariants around OPENSSL_armcap_P.

[briansmith]

No description provided.

[briansmith]

@joshlf PTAL.

[codecov]

Codecov Report

Merging #1673 (2994555) into main (901441f) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1673      +/-   ##
==========================================
- Coverage   96.04%   96.03%   -0.01%     
==========================================
  Files         133      133              
  Lines       19913    19921       +8     
  Branches      199      199              
==========================================
+ Hits        19125    19131       +6     
  Misses        752      752              
- Partials       36       38       +2     

Files	Coverage Δ
src/cpu.rs	100.00% <100.00%> (ø)
src/cpu/arm.rs	95.00% <100.00%> (+0.43%)	⬆️

... and 1 file with indirect coverage changes

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[briansmith]

Once we're done with this, I will do the same for the Intel side.

[briansmith]

Another change I need to make depends on this, so I'm going to merge it as-is. If you comment on any of the SAFETY comment stuff, I'll update them in a future PR based on your comments.

[joshlf]

@joshlf PTAL.

I don't know enough about OPENSSL_armcap_P to be able to give very precise feedback, but in broad strokes this looks like exactly the kind of reasoning that we try to encode in zerocopy 👍