[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
> 
> It is implemented in Rust: https://github.com/rust-lang/compiler-builtins/blob/acb3a0074d8fd8d9ca655a8cddef82decf0aa50d/compiler-builtins/src/int/mul.rs#L108

That implementation just uses `mul`, then `mul` has to be expanded somehow 

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