bpf, doc: also add s390x as arch to sysctl description

Looks like this was accidentally missed, so still add s390x
as supported eBPF JIT arch to bpf_jit_enable.

Fixes: 014cd0a3

 ("bpf: Update sysctl documentation to list all supported architectures")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>