Gaudi / Externals Update, master branch (2020.11.19.)

This MR updates the master branch to use atlasexternals-2.0.86 for all projects. The changes wrt. atlasexternals-2.0.85 are the following (atlasexternals@2.0.85...2.0.86):

• Various updates to make the externals build on the aarch64-centos7-gcc9-opt platform;
• Added a package for building Gaudi as part of the externals projects, building version v35r0.000 by default.

The latter update is very significant. With it we stop building Gaudi as a separate project, and move to building it as part of AthenaExternals, AthSimulationExternals, etc. Just like we've done for instance for Geant4 since we moved to CMake. This means that the build scripts of this repository no longer need to handle Gaudi as a separate project, and that we would no longer produce separate RPMs for it either.

The latter means that we will need to carefully choreograph how this MR is merged in. Since the nightly Jenkins configurations will have to be adjusted for these changes. (Pinging @aundrus and @boeriu for this.) If things go exceptionally well, I would suggest that we merge this in tomorrow evening, and build the nightlies with the new configuration over the weekend.

A note to @leggett and @fwinkl: From now on the method for switching to a new version of Gaudi will be to either

• Create a merge request in atlas/atlasexternals if the update requires changes in the atlasexternals/External/Gaudi package;
• Similar to how we set the version of LCG to use, add something like -DATLAS_GAUDI_TAG="v35r0.001" in the build_externals.sh scripts of the Gaudi based projects if the update does not require changes in the build infrastructure.

Pinging the software coordinators: @emoyse and @elmsheus.

Also pinging @clemenci for information. 😉