bpf: iterators: Split iterators.lskel.h into little- and big- endian versions

iterators.lskel.h is little-endian, therefore bpf iterator is currently
broken on big-endian systems. Introduce a big-endian version and add
instructions regarding its generation. Unfortunately bpftool's
cross-endianness capabilities are limited to BTF right now, so the
procedure requires access to a big-endian machine or a configured
emulator.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-25-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>