[CI] Add llvm-premerge-linux-32-runners runner set

This is a new runner set that runs in kubernetes-novolume mode on the libc++
node pool. This is intended to replace all three libc++ node pools as it can
run arbitrary containers specified directly in the workflows. Setting it up
as a separate nodepool for now to enable the transition and also to validate
that the konnectivity-agent changes are actually going to hold.

Reviewers: cmtice

Reviewed By: cmtice

Pull Request: https://github.com/llvm/llvm-zorg/pull/922