powerpc: Only define _TASK_CPU for 32-bit

We have some interesting code in our Makefile to define _TASK_CPU, based
on awk'ing the value out of asm-offsets.h. It exists to circumvent some
circular header dependencies that prevent us from referring to
task_struct in the relevant code. See the comment around _TASK_CPU in
smp.h for more detail.

Maybe one day we can come up with a better solution, but for now we can
at least limit that logic to 32-bit, because it's not needed for 64-bit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210418131641.1186227-1-mpe@ellerman.id.au