target-alpha: Enable the alpha-softmmu target.

With all of the pre-existing code that would not compile gone,
this is the earliest point at which the target can be enabled.

There is no machine defined yet, so this will crash on startup.
Enable the target anyway, to make sure that further compilation
problems do not creep back in.

Signed-off-by: Richard Henderson <rth@twiddle.net>
