add CI build with clang

[axel-h]

This addresses parts of https://github.com/seL4/sel4bench/issues/21 and adds a build with clang. The resulting archive gets the suffix -clang. For gcc builds no suffix is used to avoid breaking any existing workflows.

[lsf37]

This needs more work. If we change something, we need to change it both in the deployment workflow and the PR workflow. We don't want them to go out of sync and test different things.

I also don't like the suffix hack. If we do this, the compiler should be properly part of the image name. If we want to do only one run, we can add a fixed -gcc to the image download in the hardware run job. The actions themselves don't see the image name, so they won't see a difference.

[axel-h]

Making this a draft PR for now. Currently, my main motivation for having a clang build also is to see, if this reports something other than the gcc build. And to get more insight in how the github ci actions work.