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

[Alessandro Decina via llvm-commits]

alessandrod wrote:

> I don't understand how anyone could be relying on libcalls now with BPF. The current behavior is to emit a hard error on any case that would require a call.

That would require an _external_ call.

But when compiling eBPF rustc forces LTO, all the code is merged in one module and the libcalls are not external.

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