cpu: Constify CPUState::cc (cached CPUClass pointer)

Various CPUState can share the same CPUClass parent,
and must not update its fields. Protect the CPUClass
by marking the CPUState pointer const.

Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Message-ID: <20260705215729.62196-2-philmd@oss.qualcomm.com>