hw/core/cpu-sysemu: used cached class in cpu_asidx_from_attrs

hw/core/cpu-sysemu: used cached class in cpu_asidx_from_attrs

This is a heavily used function so lets avoid the cost of
CPU_GET_CLASS. On the romulus-bmc run it has a modest effect:

  Before: 36.812 s ±  0.506 s
  After:  35.912 s ±  0.168 s

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220811151413.3350684-4-alex.bennee@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220923084803.498337-4-clg@kaod.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>