selftests/bpf: Trim DENYLIST.s390x

Enable all selftests, except the 2 that have to do with the userspace
unwinding, and the new exceptions test, in the s390x CI.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919101336.2223655-11-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>