powerpc/64s: Fix GENERIC_CPU build flags for PPC970 / G5

Big-endian GENERIC_CPU supports 970, but builds with -mcpu=power5.
POWER5 is ISA v2.02 whereas 970 is v2.01 plus Altivec. 2.02 added
the popcntb instruction which a compiler might use.

Use -mcpu=power4.

Fixes: 471d7ff8 ("powerpc/64s: Remove POWER4 support")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921014103.587954-1-npiggin@gmail.com

arch/powerpc/Makefile

@@ -152,7 +152,7 @@ CFLAGS-$(CONFIG_GENERIC_CPU) += -mcpu=power8
 CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=power9,-mtune=power8)
 else
 CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=power7,$(call cc-option,-mtune=power5))
-CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mcpu=power5,-mcpu=power4)
+CFLAGS-$(CONFIG_GENERIC_CPU) += -mcpu=power4
 endif
 else ifdef CONFIG_PPC_BOOK3E_64
 CFLAGS-$(CONFIG_GENERIC_CPU) += -mcpu=powerpc64