You're telling someone in a predominately black genre of music with various collaborations under his belt, who owes his breakthrough in a large part to a founding member of N.W.A., who wrote a song to encourage voting Bush out of office, might take exception to Trump? I know some people will follow Trump off the cliff, but I can't believe anyone thought Eminem would be anything but anti-Trump.
It really shouldn't be surprising considering Eminem previously called Trump a bitch in "No Favors" earlier this year (and also Campaign Song from last year), of whom he said he would destroy their brand and explicitly told Ann Coulter to go fuck herself (That's for Bland and Philando). Say what you want about his homophobic lyrics or other problematic lyrics, Eminem has never been shy about telling racists where to stick it, Trump included.
EDIT: I just had a wonderful thought: If this gets turned into an single and charts, this will be heard everywhere in America. Racist Eminem fans will hear on a constant basis that their idol wants them to be against Trump or fuck off.