[llvm] github: Add tablegen directories to PR autolabeler (PR #109129)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LGTM. You'll need to wait for @tstellar to create the `pr-subscribers-tablegen` team before merging this though.

I thought the plan was to get rid of the pr-subscribers lists. I removed myself from all of those a while ago 

https://github.com/llvm/llvm-project/pull/109129