clang: Expose -mcpu flag

[arthurfabre]

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.