Relativize last_sp (and top_frame_sp on PowerPC) in interpreter frames.

By changing the "last_sp" member in interpreter frames from being an absolute address into an offset that is relative to the frame pointer, we don't need to change the value as we freeze and thaw frames of virtual threads. This is since we might freeze and thaw from and to different worker threads, so the absolute address to locals might change, but the offset from the frame pointer will be constant.

This subtask only handles "last_sp" (and its close equivalent "top_frame_sp" on PowerPC). The relativization of other interpreter frame members are handled in other subtasks to JDK-8289296.