libbpf: Add multi-prog section support for struct_ops

Adjust struct_ops handling code to work with multi-program ELF sections
properly.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200903203542.15944-7-andriin@fb.com