Enable -Wcpp-undef for GHC and runtime system
This gets us much of the benefit of enabling it globally, which avoiding
(at least for now) the pain of making the core libraries build as well.
See Trac #13636.
Test Plan: Validate
Reviewers: erikd, austin
Subscribers: rwbarton, thomie
GHC Trac Issues: Trac #13636
Differential Revision: https://phabricator.haskell.org/D3517