[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
* 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"

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