[llvm] [PowerPC] add PowerPC path for PR labeler (PR #82200)

[Tom Stellard via llvm-commits]

tstellar wrote:

> > Team Created: https://github.com/orgs/llvm/teams/pr-subscribers-backend-powerpc/
> 
> Thanks a lot, @tstellar . I've requested to join this team. If possible, could you make me and @stefanp-ibm as admin for this team (Stefan and I are both for IBM and actively working on PowerPC target), so we can approve anyone who wants to join without bothering you?

OK, done.

https://github.com/llvm/llvm-project/pull/82200