arch/tile: prevent corrupt top frame from causing backtracer runaway

The backtracer will normally cut itself off after 100 frames anyway,
but it's messy.  With this change we notice that the frame being
reported is the same as the last one, and cut off the dump with a
message similar to what gdb displays in the same circumstance.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>