[CI] Prune unused archs from libnccl

[hcho3]

This further reduces the size of the compiled binary.

[trivialfis]

Thank you for working on this. I think this needs to be a CI specific change. People might not want XGBoost to prune the nccl on their system.

[RAMitchell]

Agree with Jiaming that users self installing probably don't need this, and it could easily break their install process.

Is it possible to create the option PRUNE_NCCL which performs the prune then modifies the nccl library path variable in find_nccl.

[hcho3]

People might not want XGBoost to prune the nccl on their system.

The custom target is entirely optional, as it's not part of the all target. So you need to explicitly opt in: ninja PruneLibNccl. Users who install from the source will not be affected.

[hcho3]

Is it possible to create the option PRUNE_NCCL which performs the prune then modifies the nccl library path variable in find_nccl.

This is hard, as you'd need to modify the nccl path as a result of building a target.

[hcho3]

@trivialfis @RAMitchell I revised this pull request so that the changes are restricted to scripts in the CI directory tests/ci_build. Can you review again?

[RAMitchell]

Thanks. I was curious if it can be done in pure cmake with a bit less hacking, here is my attempt: #8183. Not sure if it's any better.

[RAMitchell]

Let's go with this version I think.