powerpc/64s: Enhance the information in cpu_show_spectre_v1()

We now have barrier_nospec as mitigation so print it in
cpu_show_spectre_v1() when enabled.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>