Fix the return value from _Unwind_Backtrace() on arm.

If unwinding stops due to hitting the end of the call chain, the return
value is supposed to be _URC_END_OF_STACK; other values indicate internal
errors. The return value from get_eit_entry() is now returned without
translating it to _URC_FAILURE, so that callers can see _URC_END_OF_STACK
when it happens.