devops: add s390x containers

[taronaeo]

This Pull Request adds the s390x Llama.cpp Dockerfiles, separate away from the original cpu.Dockerfile as we have additional dependencies. zDNN backend Dockerfile will be added at a later date.

GitHub Actions Workflow docker.yml will not build the s390x containers yet until I can verify that the ubuntu-24.04-s390x image is already made available to the Llama.cpp repository.

[taronaeo]

I notice that my patched OpenBLAS has slower performance for Prompt Processing but faster performance for Token Generation. This doesn't look right, I might revert back to the distributed OpenBLAS instead.

[taronaeo]

Reverted back to using libopenblas-dev as the performance difference isn't that much compared to my custom patch, and lesser maintenance cost.

model	size	params	backend	threads	test	t/s (libopenblas-openmp-dev)	t/s (libopenblas-dev)	t/s (custom OpenBLAS fork w/ cache patch)
granite 3B Q4_K - Medium	1.44 GiB	2.53 B	BLAS	8	pp512	29.02 ± 0.43	60.95 ± 0.05	55.95 ± 0.13
granite 3B Q4_K - Medium	1.44 GiB	2.53 B	BLAS	8	tg128	20.72 ± 0.01	21.89 ± 0.04	22.50 ± 0.20

Also, I've added s390x docker build to docker.yml. If it takes an unreasonable amount of time to build, I will switch it to use the actual ubuntu-22.04-s390x or ubuntu-24.04-s390x image. Re-requesting review in-case I missed anything.

Edit: My custom OpenBLAS patch is way faster without Docker. But I can't seem to replicate the same performance oh well.

model	size	params	backend	threads	test	t/s
granite 3B Q4_K - Medium	1.44 GiB	2.53 B	BLAS	8	pp512	81.98 ± 0.04
granite 3B Q4_K - Medium	1.44 GiB	2.53 B	BLAS	8	tg128	21.61 ± 0.02