[RFC] Workaround mpstat overflow

sysstat has a workaround 32bit /proc/stat, but it is outdated. More unfortunately recent /proc/stat another unreliable source and we need another workaround. 

This pull request remove a former workaround and introduce a new workaround for latter.

This is a demonstrate how ll_sp_value() make an overflow. 
https://gist.github.com/kosaki/3c3b1aff66014bac680f


Comment, feedback are welcome!