selftests/seccomp: Make seccomp tests work on big endian

The seccomp_bpf test uses BPF_LD|BPF_W|BPF_ABS to load 32-bit values
from seccomp_data->args. On big endian machines this will load the high
word of the argument, which is not what the test wants.

Borrow a hack from samples/seccomp/bpf-helper.h which changes the offset
on big endian to account for this.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -82,7 +82,13 @@ struct seccomp_data {
 };
 #endif
 
+#if __BYTE_ORDER == __LITTLE_ENDIAN
 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
+#elif __BYTE_ORDER == __BIG_ENDIAN
+#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]) + sizeof(__u32))
+#else
+#error "wut? Unknown __BYTE_ORDER?!"
+#endif
 
 #define SIBLING_EXIT_UNKILLED	0xbadbeef
 #define SIBLING_EXIT_FAILURE	0xbadface