Is porting cocos2d-x to 64 bit system an easy job?

as title, I wanna port cocos2d-x to 64bite linux

It is already ok on ubuntu 64 bit.
You can refer [[How to run HelloWorld and tests on linux]].

does it mean 64 bit compatible consideration has been included during the cocos2d-x project development?

No, we don’t consider the compatibility when developing cocos2d-x.
But it seems it is 64 bit compatible.
We just offer 64 bit lib cocos2d-x uses.