[POWERPC] Make crash.c work on 32-bit and 64-bit

To compile kexec on 32-bit we need a few more bits and pieces. Rather
than add empty definitions, we can make crash.c work on 32-bit, with
only a couple of kludges.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>