Update machine detection

[mayeut]

All unknown machines were detected as x86_64 or i686 depending on their word size.
Update the detection in order to get the untouched machine name for those arch and only consider i686 running on x86_64 to be eligible to a name change.

Example:
before, aarch64 was reported as x86_64, now aarch64

[codecov]

Codecov Report

Merging #201 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   87.43%   87.42%   -0.02%     
==========================================
  Files          19       19              
  Lines         963      962       -1     
  Branches      210      210              
==========================================
- Hits          842      841       -1     
  Misses         84       84              
  Partials       37       37

Impacted Files	Coverage Δ
auditwheel/policy/__init__.py	91.3% <100%> (-0.19%)	⬇️

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