bpf, s390: fix build for libbpf and selftest suite

The BPF feature test as well as libbpf is missing the __NR_bpf
define for s390 and currently refuses to compile (selftest suite
depends on libbpf as well). Similar issue was fixed some time
ago via b0c47807 ("bpf: Add sparc support to tools and
samples."), just do the same and add definitions.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>