It's for C. I'm asking for ASM
Which means that you'll have to implement the semantics of __builtin_expect yourself. It isn't difficult, just awkward. If the branch predictor doesn't have history for the branch then it will assume that the branch is taken when the branch is backwards and not taken when it is forward.
So you may have to restructure your code, using the opposite condition in the branch instruction and move code to accommodate that. An unconditional forward jump with a conditional backward branch is common, the way a C compiler implements a for() loop for example.