libbpf: Refactor bpf_object__reloc_code

Refactor bpf_object__append_subprog_code out of bpf_object__reloc_code
to be able to reuse it to append subprog related code for the exception
callback to the main program.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230912233214.1518551-15-memxor@gmail.com


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