gdb/arm: Don't rely on loop detection to stop unwinding

Setting SP of the next frame to the same address as the current frame
is an ugly way to stop the unwinding.  A cleaner way is to rely on
the frame_unwind_stop_reason function to return UNWIND_OUTERMOST.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>