Add x86_64_v3-centos7-clang12 architecture
This MR proposes the addition of architecture x86_64_v3-centos7-clang12, introducing x86_64_v3-centos7-clang12-opt
and x86_64_v3-centos7-clang12-dbg
in LCG_101
This MR proposes the addition of architecture x86_64_v3-centos7-clang12, introducing x86_64_v3-centos7-clang12-opt
and x86_64_v3-centos7-clang12-dbg
in LCG_101