Building ppc64 on ppc32

[Justin Hibbits]

I might be doing this wrong, but when trying to build the kernel for my G5
(powerpc64) I used the following line:

sudo make buildworld buildkernel KERNCONF=GENERIC64 TARGET_ARCH=powerpc64
TARGET=powerpc

Bug got the error:
/usr/src/gnu/lib/libssp/libssp_nonshared/../../../../contrib/gcclibs/libssp/ssp-local.c:1:
error: 64 bit CPU required

What's the correct way to cross-build for ppc64 on a ppc32?  I had done this
before a few times, but can't for the life of me remember how.

- Justin