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

arch/powerpc/Makefile

@@ -438,12 +438,15 @@ endif
 endif
 
 ifdef CONFIG_SMP
+ifdef CONFIG_PPC32
 prepare: task_cpu_prepare
 
 PHONY += task_cpu_prepare
 task_cpu_prepare: prepare0
 	$(eval KBUILD_CFLAGS += -D_TASK_CPU=$(shell awk '{if ($$2 == "TASK_CPU") print $$3;}' include/generated/asm-offsets.h))
-endif
+
+endif # CONFIG_PPC32
+endif # CONFIG_SMP
 
 PHONY += checkbin
 # Check toolchain versions: