[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

Sorry for the ping. On a second look, I need to rethink this change a little bit. While it was suggested that we tell which lib calls are available for BPF, if I simply say `__udivti3` is not available, I get a codegen error `unsupported library call operation`.

In the meantime, the best solution for my use case is still allowing BPF to emit lib calls under a sub target feature while linking them agains Rust's compiler-builtins library.

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