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

[Alessandro Decina via llvm-commits]

alessandrod wrote:

> > That implementation just uses `mul`, then `mul` has to be expanded somehow
> 
> Here: https://github.com/rust-lang/compiler-builtins/blob/acb3a0074d8fd8d9ca655a8cddef82decf0aa50d/compiler-builtins/src/int/mul.rs#L7

Again, that _specific_ implementation just says "use mul" - ie it requires the backend to be able to codegen 128 bit mul, which BPF doesn't, so you get an external libcall. 

If you enable the "c" feature you can get the compiler-rt libcall implementation, which actually implements 128 bit mul. 

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