ppc: fix AT_VECTOR_SIZE on arch/ppc

Commit 4f9a58d7 ("increase
AT_VECTOR_SIZE to terminate saved_auxv properly") changes the size of
AT_VECTOR_SIZE from hard coded '44' to a calculation based on the value
of AT_VECTOR_SIZE_ARCH and AT_VECTOR_SIZE_BASE.

The change works for arch/powerpc, but it breaks arch/ppc because the
needed AT_VECTOR_SIZE_ARCH is not present in include/asm-ppc/system.h
and a default value of 0 is used instead.  This results in
AT_VECTOR_SIZE being too small and it causes a kernel crash on loading
init.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>