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

[via llvm-commits]

yonghong-song wrote:

> 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?

In kernel, we will define (for now) bpf_unreachable() as a kfunc for bpf prog.
See
  https://lore.kernel.org/bpf/20250511182744.1806792-1-yonghong.song@linux.dev/
That means bpf_unreachable() is *reserved* for bpf infra. So users should not define bpf_unreachable() for themselves.

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