selftests/seccomp: Add powerpc support

Wire up the syscall number and regs so the tests work on powerpc.

With the powerpc kernel support just merged, all tests pass on ppc64,
ppc64 (compat), ppc64le, ppc, ppc64e and ppc64e (compat).

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>