[PPC64] xmon sparse cleanups

[Paul Mackerras]

David Gibson writes:

> Andrew, please apply:
> 
> This patch removes many sparse warnings from the xmon code.  Mostly
> K&R function declarations and 0-instead-of-NULLs.

The trouble with this patch is that it makes ppc-opc.c diverge from
the version in binutils, which is where it came from.  I'd rather keep
it as close as possible to that version.  I have no problem with the
changes to the other files.

Paul.