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

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 f48b8296b315 ("[PATCH] powerpc32: Set cpu
explicitly in kernel compiles"). This change was acceptable since the
TARGET_CPU logic was 64-bit only.

Since commit 0e00a8c9fd92 ("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: 0e00a8c9fd92 ("powerpc: Allow CPU selection also on PPC32")
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>