[RIPSim & Sim] check dhrystone is correctly finished

[khei4]

https://github.com/YosysHQ/picorv32/blob/f00a88c36eaab478b64ee27d8162e421049bcc66/dhrystone/start.S#L37-L50

We can see 'DONE' in registers?

[khei4]

I found picorv32 uses original stdlib by default, but we can use picorv32 stdlib and startup by setting flag. https://github.com/YosysHQ/picorv32/blob/f00a88c36eaab478b64ee27d8162e421049bcc66/dhrystone/Makefile#L6-L8