[llvm] github: Add llvm:mc label for generic MC interface (PR #153737)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Requests to admin:)
> 
> * Rename the `mc` label to `llvm:mc`
> * Add `issue-subscribers-llvm:mc` team
> * Delete the team [github.com/orgs/llvm/teams/pr-subscribers-mc](https://github.com/orgs/llvm/teams/pr-subscribers-mc) , the only member is @nvjle
> * Change the name of [github.com/orgs/llvm/teams/pr-subscribers-llvm-mc](https://github.com/orgs/llvm/teams/pr-subscribers-llvm-mc) to "pr-subscribers-llvm:mc"

I renamed `mc` to `llvm:mc`, but the other tasks can only be performed by an admin. @tstellar 

https://github.com/llvm/llvm-project/pull/153737