Hadrian unnecessarily builds target libraries when cross-compiling _build/stage1/target-ghc

When I specify _build/stage1/target-ghc as the hadrian target when cross compiling, hadrian should simply copy _build/stage0/target-ghc. It indeed does that in the end, but also builds a bunch of target libraries in _build/stage1, which is unnecessary, given those libraries won't be linked into the target I specify.