libbpf: Support kfunc detection in light skeleton.

Teach gen_loader to find {btf_id, btf_obj_fd} of kernel variables and kfuncs
and populate corresponding ld_imm64 and bpf_call insns.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230321203854.3035-4-alexei.starovoitov@gmail.com