Set mcpu=v3, add mcpu configurable (cmake, env var)

The version of BPF instruction set that is targeted by LLVM can be [toggled with the `mcpu` flag](https://www.kernel.org/doc/html/latest/bpf/bpf_devel_QA.html#q-new-bpf-instruction-for-kernel-and-llvm). Per @yonghong-song , internally at Meta we've seen perf wins of 5-15% from bumping to most recent instruction set version, v3.

We currently don't expose a way to change the target instruction set version in the BCC API. We should consider adding a few ways to do this:
* compile-time via cmake
* env var or similar for tools users
* config on `BPF` object BCC api exposes for application developers

We should also consider changing the default to v3. Regardless of whether that's possible, should add the configs anyways.