[llvm] [Offload] Fix PowerPC builds that pass -mcpu (PR #138327)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> @jhuber6, I'm going to merge #144372 as soon as CI passes. libcxx tests hang for unknown reason, but unlikely due to changes in the PR.

Alright, I can try to rebase that.

https://github.com/llvm/llvm-project/pull/138327