Consider adding a CI configuration that builds stage2 with -O2 for AArch64.

Summary

I recently wrote a patch that got merged, despite it turning out to be broken with -O2 on AArch64 (see #24586 (closed)).

This was only caught when colleagues complained about being unable to build ghc head locally.

I think testing this config would be beneficial. But I don't know if we have the resources to do so for every run.

Maybe there is already a label to use to enable such a config which I forgot about?