libbpf: Extract ELF processing state into separate struct

Name currently anonymous internal struct that keeps ELF-related state
for bpf_object. Just a bit of clean up, no functional changes.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211021014404.2635234-3-andrii@kernel.org