libbpf: Move elf_find_func_offset* functions to elf object

Adding new elf object that will contain elf related functions.
There's no functional change.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-9-jolsa@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>