ppc: Add ppc_set_compat_all()

ppc: Add ppc_set_compat_all()

Once a compatiblity mode is negotiated with the guest,
h_client_architecture_support() uses run_on_cpu() to update each CPU to
the new mode.  We're going to want this logic somewhere else shortly,
so make a helper function to do this global update.

We put it in target-ppc/compat.c - it makes as much sense at the CPU level
as it does at the machine level.  We also move the cpu_synchronize_state()
into ppc_set_compat(), since it doesn't really make any sense to call that
without synchronizing state.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>