samples/seccomp: fix 31 bit build on s390

On s390 the flag to force 31 builds is -m31 instead of -m32 unlike
on all (?) other architectures.

Fixes this compile error:

  HOSTCC  samples/seccomp/bpf-direct.o
cc1: error: unrecognized command line option "-m32"
make[2]: *** [samples/seccomp/bpf-direct.o] Error 1

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>