kernel: fixup KARCH for powerpc64 builds

The kernel calls both ppc64 and ppc32 "powerpc", so we need to fixup
LINUX_KARCH when building with ARCH=powerpc64.

Signed-off-by: Florian Larysch <fl@n621.de>

include/kernel.mk

@@ -85,6 +85,8 @@ else ifneq (,$(findstring $(ARCH) , armeb ))
   LINUX_KARCH := arm
 else ifneq (,$(findstring $(ARCH) , mipsel mips64 mips64el ))
   LINUX_KARCH := mips
+else ifneq (,$(findstring $(ARCH) , powerpc64 ))
+  LINUX_KARCH := powerpc
 else ifneq (,$(findstring $(ARCH) , sh2 sh3 sh4 ))
   LINUX_KARCH := sh
 else ifneq (,$(findstring $(ARCH) , i386 x86_64 ))