clang: Expose `-mcpu` flag

By default `clang` uses `-mcpu=probe`, which attempts to probe the current kernel to detect what eBPF features it can use / will be accepted by the kernel verifier.

It can also explicitly be set to `v1`, `v2` or `v3`. Expose this (or maybe a generic means of passing flags to clang) to allow users to set this. It's useful for those that compile eBPF on one machine, and run it on another.