[POWERPC] Select proper defconfig for crosscompiles

The trick for finding the right defconfig is neat, but you forgot to
provide an i686_defconfig.  ;-)

More seriously, cross compiling the defconfig is often useful, e.g. for
testing the compilation of patches that touch multiple architectures,
and this patch therefore chooses g5_defconfig if $(CROSS_COMPILE) is
non-empty.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>