The P-trap needs to be under the tub. Running that long line to it would allow crud to accumulate and it would quickly start to smell, and it would not pass code. Unless the slab is a post-tensioned slab, where cutting a cable is likely to cause the slab to fail, while a pain, it is fairly common to cut reinforced slabs to move things around. There are various ways to add back in reinforment steel prior to refilling the holes you make. If it is a tensioned slab, then you need a structural engineer.