CPU targets

[pearce8]

• Archspec CPU microarchitecture options we should link to in the docs: https://github.com/archspec/archspec-json/blob/master/cpu/microarchitectures.json
• How do packages typically specify targets - and what is best practice? --- Per @scheibelp, they typically do not
• How can we make x86_64 or x86_64_v2 or x86_64_v3 or a64fx variants per system, such that they set the target microarchitecture for the packages as appropriate?
• @scheibelp will propose an interface for one system
• We should stick with x86_64_v2 as the default per @becker33
• Why are we setting this in GitHub/workflows/run.yml: spack config add "packages:all:target:[x86_64_v3]"