fixup identical stack trace coalescing

Forgot to set the thread-specific pointer!

Also - changed stack "size" nomenclature to stack depth because it makes
more sense.

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>
only dlsym(LR) if we have a singular stack frame

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>