Discern ELF targeting eBPF for Windows from ones targeting Linux

[lmb]

Describe the feature you'd like supported

Right now it is not possible to determine the target platform from the contents of an ELF. A program compiled against Linux headers looks the same as a programs compiled against Windows ones. This is a problem for projects which need to support loading ELF from both platforms because while the ELF are superficially similar the contents aren't compatible:

• Constants like map type differ
• Section names are distinct
• Even where section names match, programs are not necessary compatible (see xdp_md should be cross-platform portable #3212)

There should be a way for toolchains to figure out the platform of an ELF.

Proposed solution

1. Add a new ELF section similar to .license which contains a string that indicates the platform, e.g. windows or linux.
2. Somehow set the ELF OS/ABI field to something meaningful.

   $ readelf -h testdata/loader-clang-el.elf 
   ELF Header:
     Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
     Class:                             ELF64
     Data:                              2's complement, little endian
     Version:                           1 (current)
     OS/ABI:                            UNIX - System V
     ABI Version:                       0
   

It would be best if this was coordinated with upstream libbpf somehow.

Additional context

No response

[lmb]

I'll send a PR which adds a temporary workaround to one of the headers in this project. I won't do the upstream discussion how to make this cross platform.