samples/seccomp/Makefile: do not build tests if cross-compiling for MIPS

The Makefile is designed to use the host toolchain so it may be unsafe
to build the tests if the kernel has been configured and built for
another architecture.  This fixes a build problem when the kernel has
been configured and built for the MIPS architecture but the host is not
MIPS (cross-compiled).  The MIPS syscalls are only defined if one of the
following is true:

 1) _MIPS_SIM == _MIPS_SIM_ABI64
 2) _MIPS_SIM == _MIPS_SIM_ABI32
 3) _MIPS_SIM == _MIPS_SIM_NABI32

Of course, none of these make sense on a non-MIPS toolchain and the
following build problem occurs when building on a non-MIPS host.

  linux/usr/include/linux/kexec.h:50: userspace cannot reference function or variable defined in the kernel
  samples/seccomp/bpf-direct.c: In function `emulator':
  samples/seccomp/bpf-direct.c:76:17: error: `__NR_write' undeclared (first use in this function)

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>