[POWERPC] Prepare for spu disassembly in xmon

In order to do disassembly of spu binaries in xmon, we need to abstract
the disassembly function from ppc_inst_dump.

We do this by making the actual disassembly function a function pointer
that we pass to ppc_inst_dump(). To save updating all the callers, we
turn ppc_inst_dump() into generic_inst_dump() and make ppc_inst_dump()
a wrapper which always uses print_insn_powerpc().

Currently we pass the dialect into print_insn_powerpc(), but we always
pass 0 - so just make it a local.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>