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>