refactor: rename each avx2 ref file to x86_64_v3 and create symlink for avx2

Prepare the Moore tests green for the new x86_64_v{3,4} platforms (now in lhcb-lcg-test, soon everywhere)

Done with e.g.:

find -name '*.x86_64+avx2+fma-opt' -exec bash -c 'new={}; tmp=${new/x86_64+avx2+fma-opt/x86_64_v3-opt} ref=${tmp#./}; ln -s $new $ref' \;        

fyi: @rmatev @jonrob

I don't see how this could interfere with any of the tests on the working platforms,
so I'm assigning lhcb-lcg-test to test it since we don't have these platform in the ci-tests.
@jonrob or @gawang, if that is not ok please feel free to remove the label again