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

[via llvm-commits]

yonghong-song wrote:

> How does the resulting verifier error look like?

The current verifier error looks like
```
  func#0 @0
  last insn is not an exit or jmp
```

This is due to that there is a logic in verifier to ensure the last insn in the func must be an 'exit' or 'jmp' insn. If this approach sounds reasonable, if __unreachable_helper is the last insn, the verifier log can be changed to e.g.,
    last insn is marked as unreachable, maybe due to uninitialized variable?
Such more explicit message will prompt users to check their auto variables.

Without this patch, the user will simply get error message
   last insn is not an exit or jmp
and users needs to dig into asm codes to find why the code is generated that way.

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