add new v3 opt+g platforms for the throughput tests

I would like to add these platforms to start building opt+g in the nightlies as default platform for the throughput tests.

LCG_100/x86_64_v3-centos7-gcc10-opt+g.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.                                       #
+###############################################################################
+include_guard(GLOBAL)
+
+set(LHCB_PLATFORM x86_64_v3-centos7-gcc10-opt+g)
+set(LCG_ARCHITECTURE x86_64_v3)
+set(LCG_OPTIMIZATION "opt+g" )
+include(${CMAKE_CURRENT_LIST_DIR}/x86_64-centos7-gcc10-opt.cmake)
+