arch/powerpc/kernel/crash.c isn't safe for PPC32 (yet?), so don't build it.

Built with CONFIG_KEXEC=y for pmac32_defconfig, pseries_defconfig,
and g5_defconfig.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>