Add AMD CI runners with their own performance series #410

Summary

CI's microVM and platform runners are Azure VMs with Intel Xeon CPUs (Ice Lake-SP or Emerald Rapids). So no CI job exercises an AMD CPU profile or the AMD paths of the backends, and the performance gate covers only Intel hosts. #396 deferred AMD runners (plan item 7), and doc/benchmarks.md says how one would get its own series.

The built-in AMD profiles now serve the CPUs of Azure's AMD VMs: Milan, Genoa, and Turin. #412 qualified all three on KVM in GitHub-hosted Actions runners, which are such VMs, with the full default test-microvm suite. GitHub-hosted runners can't hold a performance series, though: each job lands on any of those CPUs, or on an Intel one.

Plan

• Provision self-hosted AMD runners for the backends that CI covers, one AMD VM type each, so that each series measures one CPU:
  • KVM and MSHV on Azure Linux VMs with nested virtualization;
  • WHP on Windows VMs.
• Route the microVM test and platform jobs to them, and give them their own performance series, with an -amd suffix such as linux-kvm-virtual-machine-amd, as doc/benchmarks.md describes.
• Record their baselines from dev runs before the performance gate compares them.