selftests/bpf: fix attach_probe on s390

attach_probe test fails, because it cannot install a kprobe on a
non-existent sys_nanosleep symbol.

Use the correct symbol name for the nanosleep syscall on 64-bit s390.
Don't bother adding one for 31-bit mode, since tests are compiled only
in 64-bit mode.

Fixes: 1e8611bb ("selftests/bpf: add kprobe/uprobe selftests")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>