powerpc: Enable seccomp architecture tracking

To enable seccomp constant action bitmaps, we need to have a static
mapping to the audit architecture and system call table size. Add these
for powerpc.

__LITTLE_ENDIAN__ is used here instead of CONFIG_CPU_LITTLE_ENDIAN
to keep it consistent with asm/syscall.h.

Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/0b64925362671cdaa26d01bfe50b3ba5e164adfd.1605101222.git.yifeifz2@illinois.edu