add BPF GCC support for BPF CI

[cupermir]

This pull requests enables BPF GCC compilation testing for the CI.

Required changes in libbpf/ci where pull requested in libbpf/ci#144

[chantra]

Overall, LGTM. Did not do a throughout review, but just dropping a few comment regarding the failed set_matrix step.

Also, please do this PR against https://github.com/kernel-patches/vmtest/ instead. kernel-patches/bpf pulls the kernel from upstream and then copies kernel-patches/vmtest into it.

[chantra]

this is what seems to cause the set_matrix step failure.

The error is:

Traceback (most recent call last):
  File "/tmp/work/bpf/bpf/.github/scripts/matrix.py", line 70, in <module>
    @dataclasses.dataclass
     ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/dataclasses.py", line 1268, in dataclass
    return wrap(cls)
           ^^^^^^^^^
  File "/usr/lib/python3.12/dataclasses.py", line 12[5](https://github.com/kernel-patches/bpf/actions/runs/10881506538/job/30190585441?pr=7737#step:3:6)8, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/dataclasses.py", line 994, in _process_class
    cls_fields.append(_get_field(cls, name, type, kw_only))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/dataclasses.py", line [8](https://github.com/kernel-patches/bpf/actions/runs/10881506538/job/30190585441?pr=7737#step:3:9)52, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class '__main__.Toolchain'> for field bpf_toolchain is not allowed: use default_factory

From https://stackoverflow.com/a/75677129

it seems https://gist.github.com/chantra/b4ac937f0f0e909d238bb03f7aacae49 would work.

[kernel-patches-daemon-bpf]

Automatically cleaning up stale PR; feel free to reopen if needed