Cmm: Add support for undefined unwinding statements

And use to mark `stg_stack_underflow_frame`, which we are unable to
determine a caller from.

To simplify parsing at the moment we steal the `return` keyword to
indicate an undefined unwind value. Perhaps this should be revisited.

Reviewers: scpmw, simonmar, austin, erikd

Subscribers: dfeuer, thomie

Differential Revision: https://phabricator.haskell.org/D2738