powerpc: Merge xmon

The merged version follows the ppc64 version pretty closely mostly,
and in fact ARCH=ppc64 now uses the arch/powerpc/xmon version.
The main difference for ppc64 is that the 'p' command to call
show_state (which was always pretty dodgy) has been replaced by
the ppc32 'p' command, which calls a given procedure (so in fact
the old 'p' command behaviour can be achieved with 'p $show_state').

Signed-off-by: Paul Mackerras <paulus@samba.org>