Add PowerPC 32-bit guest memory dump support

Add PowerPC 32-bit guest memory dump support

This patch extends support for the `dump-guest-memory` command to the
32-bit PowerPC architecture. It relies on the assumption that a 64-bit
guest will not dump a 32-bit core file (and vice versa).

[dwg: I suspect this patch won't cover all cases, in particular a
32-bit machine type on a 64-bit qemu build.  However, it does strictly
more than what we had before, so might as well apply as a starting
point]

Signed-off-by: Mike Nawrocki <michael.nawrocki@gtri.gatech.edu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>