You don't even need the Arm machine for CI, with docker multi-arch builds you can build on x86 -- https://community.arm.com/developer/research/b/articles/post...

My blog post on using docker-in-docker combined with qemu-user-static and Docker BuildX to enable you to build multiple-architecture (arm, arm64, power, etc.) images using GitLab's own x86 CI cluster.

More details in the upstreamed binutils support: https://sourceware.org/ml/binutils/2016-08/msg00166.html