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

[Alessandro Decina via llvm-commits]

alessandrod wrote:

> The first version of this PR proposed a set removing only the i128 related functions, but there was a recommendation against that, since we would be hitting the error in codegen for all external calls

This is because you're not enabling the "c" feature of compiler-builtins, which links to compiler-rt, which links __multi3, then you have the definition and it's not an external call

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