[llvm] [Github] Add llvm-exegesis PR subscribers team (PR #65199)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

@tstellar It also seems like there's a `pr-subscribers-llvm-exegesis` team in addition to the `pr-subscribers-exegesis` team that is used here. Seems like `pr-subscribers-llvm-exegesis` should probably be deleted?

https://github.com/llvm/llvm-project/pull/65199