Make it possible to test fallback implementation on non-x86, non-arm system
Make it possible to test fallback implementation on non-x86, non-arm system

This simplifies the work for packager who can then package and run tests for
xsimd on any platform, whether it's supported or not.

JohanMabille/xsimd,jmabille/nxsimd,xtensor-stack/xsimd,JohanMabille/xsimd,xtensor-stack/xsimd