[llvm] [BPF] Allow libcalls behind a feature gate (PR #168442)

[via llvm-commits]

https://github.com/yonghong-song commented:

Please explain how bpf program handles builtin function like __multi3() in rust environment. Currently the builtin function like __multi3() won't work for libbpf and kernel as there is no actual function for __multi3() in final bpf program.

https://github.com/llvm/llvm-project/pull/168442