selftests/bpf: fix s930 -> s390 typo

Also check for __s390__ instead of __s390x__, just in case bpf_helpers.h
is ever used by 32-bit userspace.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>