bpf: doc: correct arch list for supported eBPF JIT

aarch64 and s390x support eBPF JIT too, correct document to reflect this and
avoid any confusion.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>