powerpc/32: Move the old 6xx -mcpu logic before the TARGET_CPU logic

The code:

  ifdef CONFIG_6xx
  KBUILD_CFLAGS          += -mcpu=powerpc
  endif

was added in 2006 in commit f48b829 ("[PATCH] powerpc32: Set cpu
explicitly in kernel compiles"). This change was acceptable since the
TARGET_CPU logic was 64-bit only.

Since commit 0e00a8c ("powerpc: Allow CPU selection
also on PPC32") this logic is no longer acceptable after the TARGET_CPU
specific. It currently appends -mcpu=powerpc at the end of the command
line, after any TARGET_CPU specific:

  gcc -Wp,-MD,init/.do_mounts.o.d ...
    -mcpu=powerpc -mbig-endian -m32 ...
    -mcpu=e300c2 ...
    -mcpu=powerpc ...
    ../init/do_mounts.c

Fixes: 0e00a8c ("powerpc: Allow CPU selection also on PPC32")
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Makefile

@@ -30,6 +30,10 @@ endif
 endif
 endif
 
+ifdef CONFIG_PPC_BOOK3S_32
+KBUILD_CFLAGS		+= -mcpu=powerpc
+endif
+
 ifeq ($(CROSS_COMPILE),)
 KBUILD_DEFCONFIG := $(shell uname -m)_defconfig
 else
@@ -241,10 +245,6 @@ KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
 # often slow when they are implemented at all
 KBUILD_CFLAGS		+= $(call cc-option,-mno-string)
 
-ifdef CONFIG_PPC_BOOK3S_32
-KBUILD_CFLAGS		+= -mcpu=powerpc
-endif
-
 cpu-as-$(CONFIG_4xx)		+= -Wa,-m405
 cpu-as-$(CONFIG_ALTIVEC)	+= $(call as-option,-Wa$(comma)-maltivec)
 cpu-as-$(CONFIG_E200)		+= -Wa,-me200