Add LCG 100

for the moment this includes only LCG 100rc2 for x86_64-centos7-gcc10-opt, 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)
• geant4 (no problem observed yet, just for precaution)

LCG_100/haswell-centos7-gcc10-opt.cmake

+###############################################################################
+# (c) Copyright 2021 CERN for the benefit of the LHCb Collaboration           #
+#                                                                             #
+# This software is distributed under the terms of the GNU General Public      #
+# Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
+#                                                                             #
+# In applying this licence, CERN does not waive the privileges and immunities #
+# granted to it by virtue of its status as an Intergovernmental Organization  #
+# or submit itself to any jurisdiction.                                       #
+###############################################################################
+
+# DO NOT EDIT: File generated by new_lcg_version.py
+
+# Make sure the toolchain is included only once
+# (to avoid double changes to the environment)
+include_guard(GLOBAL)
+
+set(LHCB_PLATFORM haswell-centos7-gcc10-opt)
+set(LCG_ARCHITECTURE haswell)
+include(${CMAKE_CURRENT_LIST_DIR}/x86_64-centos7-gcc10-opt.cmake)