target-alpha: Avoid leaking the alarm timer over reset

Move the timer from CPUAlphaState to AlphaCPU to avoid the pointer being
zero'ed once we implement reset. Would cause a segfault in
sys_helper.c:helper_set_alarm().

This also simplifies timer initialization in Typhoon.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Richard Henderson <rth@twiddle.net>
