Fix special platforms delegation

The toolchain files that create platform aliases for other platforms (like x86_64_v2-centos7-gcc10-opt is an alias to LCG x86_64-centos7-gcc10-opt), cannot be aliased as it was done for the special gcc10+dd4hep cases.

This MR fixes the LCG 101 gcc10+dd4hep aliases and modifies the generator script to allow this kind of double aliasing in the future.

I also updated the generator to include the fix added manually in !72 (merged).

assigned to @clemenci

/cc @rmatev this should fix the issue with DD4hep builds in lhcb-gaudi-head

mentioned in commit a07caadf

merged