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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> At the same time, I (an external contributor, not a maintainer) don't want to be caught in the middle of an unpleasant discussion about what should be done, like what has happened in this PR.
> 
> My kind suggestion is that you write a comment in #199542 explaining what is wrong there. This way, you'll get attention from both the author and the BPF target maintainers about your vision. Once there is consensus, I'm happy to address the changes.

I mean I think we already went through this the last time the libacll feature was added. That PR has a bunch of hardcode string names, and doesn't fundamentally address anything. This PR was fine to go as it was, and just needs a rebase 

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