libbpf, xsk: Add libbpf_smp_store_release libbpf_smp_load_acquire

Now that the AF_XDP rings have load-acquire/store-release semantics,
move libbpf to that as well.

The library-internal libbpf_smp_{load_acquire,store_release} are only
valid for 32-bit words on ARM64.

Also, remove the barriers that are no longer in use.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210305094113.413544-3-bjorn.topel@gmail.com