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

[Lucas Ste via llvm-commits]

LucasSte wrote:

If we can merge this PR, I can remove the sub target feature I created in #168442 or even revert those changes altogether.

cc. @yonghong-song and @arsenm

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