Fix stacktraces for :elixir.eval_forms/4

:erl_eval and :eval_bits can call :erlang.raise/3 with an incomplete
stacktrace, so extend the stacktrace when this occurs.

Mailing list thread:
https://groups.google.com/forum/#!topic/elixir-lang-core/cpSlgBjrRBQ

Edit: This needs a test to prevent regression.