Add LCG 99 toolchains
for the moment this includes only gcc10 and clang10, and the packages are picked up from /cvmfs/sft.cern.ch.
I also included some changes to the Python script used to generate the toolchain files, as we need to exclude some LCG provided packages on top of the already excluded ones (Gaudi, CMake and Ninja):
- xenv (for consistency with old style toolchains)
- git (introduced in LCG 99, see LbDevTools!161 (merged))
- hepmc3 (dictionaries conflict with RELAX, see https://its.cern.ch/jira/browse/LBCORE-1935)