libbpf: Add "iter.s" section for sleepable bpf iterator programs

This adds a new bpf section "iter.s" to allow bpf iterator programs to
be sleepable.

Signed-off-by: Kenny Yu <kennyyu@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220124185403.468466-4-kennyyu@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>