[clang] [llvm] [BPF] Make -mcpu=v3 as the default (PR #107008)

[Yuval Deutscher via llvm-commits]

yuvald-sweet-security wrote:

> > Hey,
> > I didn't see this change on the [LLVM 20.1.0 Release Notes](https://releases.llvm.org/20.1.0/docs/ReleaseNotes.html) - it would be nice if you could add those in the future as a heads up.
> > Anyway, I got here after tracing a regression in eBPF codegen introduced in Clang 20 which results in dramatically larger kernel verifier states (and eventually causes the verifier to give up with sufficiently large functions). The culprit does indeed seem to be BPF v3 - my guess is that branching according to 32-bit registers provides less refutability for the verifier which ends up carrying more possible states as a result, but I didn't dive too deep into this.
> > FYI
> 
> The llvm patch link to add release note in llvm20: #131691

Thank you very much :)

https://github.com/llvm/llvm-project/pull/107008