powerpc: Fix defconfig choice logic when cross compiling

Our logic for choosing defconfig doesn't work well in some situations.

For example if you're on a ppc64le machine but you specify a non-empty
CROSS_COMPILE, in order to use a non-default toolchain, then defconfig
will give you ppc64_defconfig (big endian):

  $ make CROSS_COMPILE=~/toolchains/gcc-8/bin/powerpc-linux- defconfig
  *** Default configuration is based on 'ppc64_defconfig'

This is because we assume that CROSS_COMPILE being set means we
can't be on a ppc machine and rather than checking we just default to
ppc64_defconfig.

We should just ignore CROSS_COMPILE, instead check the machine with
uname and if it's one of ppc, ppc64 or ppc64le then use that
defconfig. If it's none of those then we fall back to ppc64_defconfig.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Makefile

@@ -34,11 +34,10 @@ ifdef CONFIG_PPC_BOOK3S_32
 KBUILD_CFLAGS		+= -mcpu=powerpc
 endif
 
-ifeq ($(CROSS_COMPILE),)
-KBUILD_DEFCONFIG := $(shell uname -m)_defconfig
-else
-KBUILD_DEFCONFIG := ppc64_defconfig
-endif
+# If we're on a ppc/ppc64/ppc64le machine use that defconfig, otherwise just use
+# ppc64_defconfig because we have nothing better to go on.
+uname := $(shell uname -m)
+KBUILD_DEFCONFIG := $(if $(filter ppc%,$(uname)),$(uname),ppc64)_defconfig
 
 ifdef CONFIG_PPC64
 new_nm := $(shell if $(NM) --help 2>&1 | grep -- '--synthetic' > /dev/null; then echo y; else echo n; fi)