Enabling Sustainable ARM Support for the Belle II Software

ARM-based processors are becoming increasingly common in HPC environments, as in the AWS Graviton, Fujitsu A64FX (one of the world's fastest supercomputers), and various aarch64 nodes at European computing centres. If basf2 can't build on these systems, it's locked out of an excellent infrastructure that the experiment might benefit from. The goal of this project is to explore and evaluate modern, sustainable alternatives to the current Makefile-based approach for externals, ones that can support both x86 and ARM in a unified, maintainable way without duplicating large sections of build logic.