[RFC] Add CI builder for AIX

We are planning to make our powerpc64-ibm-aix to be a tier-2 target. However, one obstacle is Github Actions is lacking neither powerpc64 nor AIX support. Considering AIX's toolchain is not cross-compile awared, we can't generate linked XCOFF binaries and libraries on X86 either.
For above reason, we propose 2 candidate solutions.

  1. Run QEMU to emulate AIX OS on Github Actions's linux container and build Rust toolchains for AIX.
  2. Integrate buildkite into rust's current workflow, since IBM OpenXL team already has an AIX agent for buildkite.

For 1, I'm worried about the efficiency of the simulation.
For 2, native machine's performance is decent but may complex current workflow.

We also considered self-hosted runner of Github Actions, however the runner application is written with c#, we don't have .NET support on AIX right now.

For 1, will IBM provide a license which allows publicly publishing an AIX image?

I'm not sure. I'm consulting internal team about it.
Personally, I prefer solution 2 since it's more efficient. I notice tier-2 target has build speed requirement, solution 1 might not meet this requirement.