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

[via llvm-commits]

yonghong-song 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.

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