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

[Alexey Bader via llvm-commits]

bader wrote:

> > Would it make sense to strip _all_ default compilation flags and explicitly specify the desired ones only?
> 
> The proper solution is #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 #144372 I'm going to assume that the community is more in favor of my approach.

@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.

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