[Pre-RFC] BTF relocations

It's actually quite common to have task->se.vruntime accesses in C BPF programs, so supporting this from the get go would be essential to make this useful for real-world applications.

Re: naming. "preserve_access_index" naming we use in C is quite opaque and not very meaningful. Everyone just remembered and use it without asking questions, but I always felt like having it named as "btf_relocatable" or something along those lines ("relocatable" is the key here, IMO) would be best. Just my 2 cents on naming.