backtrace: show call-sites instead of return-sites

I think this is more useful. When I analyze a backtrace I want to see
a call-chain rather than return-chain. The return addres may lay
inside an inlined function and have little to do with the
call-site. Example follows showing addresses translated by addr2line.

Before:

  tcp_net_channel_packet
  std::__atomic_base<unsigned int>::load(std::memory_order)
  operator()

After:

  tcp_net_channel_packet
  std::function<void (mbuf*)>::operator()(mbuf*)
  tcp_flush_net_channel

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>