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

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eddyz87 wrote:

@yonghong-song ,

> BTW, just updated a new version to incorporate some of your changes. Please take a look!

The changes lgtm, I also tried end-to-end by adjusting your kernel series to `__bpf_unreachable` and inserting `__builtin_trap()` call in one of the C tests. Verifier reports error as expected.

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