Add toolchain for x86_64+avx2+fma-centos7-gcc9-opt
this is needed to fix the failing builds in the nightlies, e.g. https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master-ref/1352/Gaudi/x86_64%2Bavx2%2Bfma-centos7-gcc9-opt/build
Edited by Rosen Matev