Use native flag in CI to not fall into VCL's emulation branch

.gitlab-ci.yml

@@ -4,6 +4,7 @@ variables:
   GIT_SUBMODULE_STRATEGY: recursive
   HYPERTHREADING: "false"
   OMP_NUM_THREADS: 1
+  DUNECI_CXXFLAGS: "-march=native"
 
 before_script:
   - echo "Running make concurrently with ${DUNECI_PARALLEL} cores, where possibly"