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>

samples/seccomp/Makefile

@@ -18,14 +18,22 @@ HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
 bpf-direct-objs := bpf-direct.o
 
 # Try to match the kernel target.
-ifeq ($(CONFIG_64BIT),)
-HOSTCFLAGS_bpf-direct.o += -m32
-HOSTCFLAGS_dropper.o += -m32
-HOSTCFLAGS_bpf-helper.o += -m32
-HOSTCFLAGS_bpf-fancy.o += -m32
-HOSTLOADLIBES_bpf-direct += -m32
-HOSTLOADLIBES_bpf-fancy += -m32
-HOSTLOADLIBES_dropper += -m32
+ifndef CONFIG_64BIT
+
+# s390 has -m31 flag to build 31 bit binaries
+ifndef CONFIG_S390
+MFLAG = -m32
+else
+MFLAG = -m31
+endif
+
+HOSTCFLAGS_bpf-direct.o += $(MFLAG)
+HOSTCFLAGS_dropper.o += $(MFLAG)
+HOSTCFLAGS_bpf-helper.o += $(MFLAG)
+HOSTCFLAGS_bpf-fancy.o += $(MFLAG)
+HOSTLOADLIBES_bpf-direct += $(MFLAG)
+HOSTLOADLIBES_bpf-fancy += $(MFLAG)
+HOSTLOADLIBES_dropper += $(MFLAG)
 endif
 
 # Tell kbuild to always build the programs