I can't say for sure that this is a kernel source problem - I'm
cross-compiling, and I've been absolutely PLAGUED with troubles and
nonworking things - but it seems to me that kern_exit1.c calls memcpy and
yet memcpy is never defined.  I solved the problem by replicating
arch/powerpc/powerpc/bcopy.c as memcpy.c and memmove.c, adding the
necessary lines to makefiles, and #define MEMCOPY at the top of one and
#define MEMMOVE at the top of the other.

Am I seeing something wrong or just another bug in my setup?