Drop -march=native being a default flag when building xeus-cpp

[mcbarton]

When building xeus-cpp -march=native shouldn't be a default flag, due to cases where xeus-cpp is cross compiled from one platform for a different platform. For example the osx 86 conda-forge package is built on a osx arm platform.

[vgvassilev]

LGTM!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.25%. Comparing base (12c4570) to head (97c8d00).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   81.25%   81.25%           
=======================================
  Files          21       21           
  Lines         864      864           
  Branches       78       78           
=======================================
  Hits          702      702           
  Misses        162      162           

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