To implement -fomit-leaf-frame-pointer, there are 2 places where we need
to check whether we have to use a frame chain (since register allocation
may allocate LR in a leaf function that omits the frame pointer, but if
LR is spilled we must emit a frame chain). To simplify this do not force
frame_pointer_needed via aarch64_frame_pointer_required, but enable the
frame chain in aarch64_layout_frame. Now aarch64_frame_pointer_required
can be removed and aarch64_can_eliminate is simplified.