Require 'libdw' for '--enable-dwarf-unwind'

This causes './configure --enable-dwarf-unwind' to exit with a helpful
error message when 'libdw' cannot be found (compared to the previous
behaviour of silently pretending the user hadn't requested DWARF support
at all).

Test Plan: ./configure --enable-dwarf-unwind # on systems with/without
libdw

Reviewers: bgamari, nh2

Reviewed By: nh2

Subscribers: nh2, rwbarton, erikd, carter

GHC Trac Issues: #15968

Differential Revision: https://phabricator.haskell.org/D5424