Use sha2 extensions on ARM by default if available

[macladson]

Use sha2 extensions on ARM by default.

Previously we would only attempt to use these if the sha2 feature was explicitly enabled.
Now, sha2 is enabled by default for ARM and only falls back to ring if no sha2 extensions are available.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.77%. Comparing base (52177cf) to head (44c44de).

Files with missing lines	Patch %	Lines
src/lib.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   81.60%   79.77%   -1.84%     
==========================================
  Files           3        3              
  Lines          87       89       +2     
==========================================
  Hits           71       71              
- Misses         16       18       +2     

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