[llvm] [BPF] Define empty set of BPF libcalls (PR #169537)

[Lucas Ste via llvm-commits]

LucasSte wrote:

> * @LucasSte Looks like you intend to use the new mechanism (based on RuntimeLibcalls.td) and to revert the previous implementation. If this is the case, the pull request should revert the previous implementation and then add new implementation. You can have two separate commits in your pull request. Also, please answer @alessandrod question above.

Hi @yonghong-song, thanks for circling back to me. For more organization, I had opened a PR to revert my previous change https://github.com/llvm/llvm-project/pull/169733 last week.

Besides this PR, I'll open one more to write `setMaxDivRemBitWidthSupported(64);` in `BPFIselLowering.cpp` so that we can lower i128 divisions correctly.

Let's wait one or two days to see if there isn't any other comment or concern with this approach. If nothing else comes up, I would suggest you to merge https://github.com/llvm/llvm-project/pull/169733 first, and then this one (after rebase).

https://github.com/llvm/llvm-project/pull/169537