panic, crash_core, sys_info: Introduce crash_kexec_in_memory_sys_info

新增crash_kexec_in_memory_sys_info：__crash_kexec()在保存寄存器/vmcoreinfo后、machine_crash_shutdown()前把sys_info()写入printk环形缓冲区，规避post_notifier看门狗风险并在vmcore保存失败时保留诊断。