alpha: replace cpu_alpha_init() with cpu_generic_init()

cpu_alpha_init() used to provide default fallback if invalid
(i.e. non existent) cpu_model were provided.

dp264 machine provides its own default so sole user of fallback
is [bsd|linux]-user targets which specifies 'any' cpu model that
fallbacks to "ev67" in cpu_alpha_init(). Push fallback handling
into alpha_cpu_class_by_name() and replace cpu_alpha_init() with
cpu_generic_init().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1503592308-93913-10-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
