Add aws-plcuster[-aarch64] stacks. These stacks build packages defined in
https://github.com/spack/spack-configs/tree/main/AWS/parallelcluster
They use a custom container from https://github.com/spack/gitlab-runners which
includes necessary ParallelCluster software to link and build as well as an
upstream spack installation with current GCC and dependencies.
Intel and ARM software is installed and used during the build stage but removed
from the buildcache before the signing stage.
Files `configs/linux/{arch}/ci.yaml` select the necessary providers in order to
build for specific architectures (icelake, skylake, neoverse_{n,v}1).
125c20bc06