[llvm] [BPF] Handle unreachable with a kfunc call (PR #131731)

[Yingchi Long via llvm-commits]

https://github.com/inclyc commented:

I can do a quick search on GitHub and find existing examples that generate `__bpf_unreachable();`. Could the `call` we generate conflict with existing ones / user defined ones?

https://github.com/llvm/llvm-project/pull/131731