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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Would it make sense to strip _all_ default compilation flags and explicitly specify the desired ones only?

The proper solution is https://github.com/llvm/llvm-project/pull/136729 but this was delayed endlessly by https://discourse.llvm.org/t/rfc-llvm-policy-for-top-level-directories-and-language-runtimes/86143. Given the PR at https://github.com/llvm/llvm-project/pull/144372 I'm going to assume that the community is more in favor of my approach. 

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