Execute and test CO-RE relocations of bpf2bpf subprogs

[ti-mo]

btf: include CO-RE relocations of bpf2bpf subprograms

ProgramSpecs read from an ELF contain the full instruction streams from the
bpf2bpf subprograms they call. When marshaling a program to be inserted into
the kernel, FuncInfos and LineInfos are collected from all called subprogs,
but CO-RE relocations were notably skipped after the linker rework in 9d739bd.

This commit gathers CO-RE relos for all subprogs included in the final
instruction stream and modifies their offsets to point to their new locations.

btf: add CO-RE read test that fails unless relocations were correctly applied

The linker rework in 9d739bd caused a piece of code that appended CORERelos
to the entrypoint instruction stream to be forgotten. The existing CO-RE tests
were all written in a way where the BPF programs would return 0 (success) if
no CO-RE relocations were applied at all.

This caused CO-RE relocations against functions (subprogs) that were only
included by a bpf2bpf call to be skipped.

This patch adds a broken test program that returns non-zero if left unaltered,
and requires CO-RE relocation against another BTF blob to return 0.
Additionally, the checks are performed from a non-inlined subprogram to ensure
CO-RE relocations against subprogs work.

---

@eiffel-fl could you verify if this solves your problem?

Fixes #586

[eiffel-fl]

Hi.

I tested it with the initial tool I was testing and specifying __attribute((noinline)) and it worked fine!
I also take a quick look to your code and it is cool!
Thank you for this :D :D :D

If you need test or review or anything, feel free to ping!

Best regards.