samples/seccomp: do not compile when cross compiled

samples/seccomp relies on the host setting which is not suitable for
crosscompilation and it actually fails when crosscompiling s390 and
powerpc all{yes,mod}config on x86_64 with

samples/seccomp/bpf-helper.h:135:2: error: #error __BITS_PER_LONG value unusable.
 #error __BITS_PER_LONG value unusable.
  ^
In file included from samples/seccomp/bpf-fancy.c:13:0:
samples/seccomp/bpf-fancy.c: In function ‘main’:
samples/seccomp/bpf-fancy.c:38:11: error: ‘__NR_exit’ undeclared (first use in this function)
   SYSCALL(__NR_exit, ALLOW),

and many others. I am doing these for compile testing and it's been
quite useful to catch issues. Crosscompiling sample code on the other
hand doesn't seem all that important so it seems like the easiest way to
simply disable samples/seccomp when crosscompiling.

Fixing this properly is not that easy as Kees explains:
: IIRC, one of the problems is with build ordering problems: the kernel
: headers used by the samples aren't available when cross compiling.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Kees Cook <keescook@chromium.org>