Add special platforms to build LHCb with DD4hep
At PAC we agreed to have special platforms that implicitly enable the build against DD4hep.
This MR adds two virtual platforms to the LCG_100 set (those used in the lhcb-dd4hep slot):
- x86_64_v2-centos7-gcc10+dd4hep-opt
- x86_64_v2-centos7-gcc10+dd4hep-dbg