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

[Yuval Deutscher via cfe-commits]

yuvald-sweet-security wrote:

@4ast perhaps you know who in LLVM can help with adding a "Changes to the BPF Backend" section to the release notes, similar to how other backends have their own sections?

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