Libdw: Add libdw-based stack unwinding

This adds basic support to the RTS for DWARF-assisted unwinding of the
Haskell and C stack via libdw. This only adds the infrastructure;
consumers of this functionality will be introduced in future diffs.

Currently we are carrying the initial register collection code in
Libdw.c but this will eventually make its way upstream to libdw.

Test Plan: See future patches

Reviewers: Tarrasch, scpmw, austin, simonmar

Reviewed By: austin, simonmar

Subscribers: simonmar, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1196

GHC Trac Issues: #10656