target-ppc: Move CPU aliases out of translate_init.c

Move array of CPU aliases to cpu-models.c, alongside model definitions.
This requires to zero-terminate the aliases array since ARRAY_SIZE() can
no longer be used in translate_init.c then.

Suggested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>