ARM Updates, master branch (2020.10.27.)

These are some updates that I had to do to make AthSimulationExternals build successfully on techlab-arm64-thunderx2-01.cern.ch against the LCG nightly picked up from /cvmfs/sft-nightlies.cern.ch/lcg/nightlies/devARM/.

Note that some of these changes may become unnecessary down the line, like the change in PyModules. But I tried to implement the changes such that they would not interfere on platforms that don't experience the limitations that aarch64-centos7-gcc8-opt does at the moment.

This is to address the issue(s) discussed in ATLINFR-3794.