cpu::intel: Do CPU feature detection in Rust.

[briansmith]

It's easiest to review this PR commit-by-commit.

[codecov]

Codecov Report

Attention: Patch coverage is 89.74359% with 12 lines in your changes missing coverage. Please review.

Project coverage is 96.59%. Comparing base (0c128bf) to head (272682e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cpu/intel.rs	89.74%	10 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2439      +/-   ##
==========================================
+ Coverage   96.58%   96.59%   +0.01%     
==========================================
  Files         180      179       -1     
  Lines       21737    21722      -15     
  Branches      538      523      -15     
==========================================
- Hits        20995    20983      -12     
- Misses        626      635       +9     
+ Partials      116      104      -12     

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[briansmith]

As codecov points out, and the comment notes, these lines are unreachable in unpatched versions of ring. (Some people do patch it in ways that make these lines reachable.)