Herbert Valerio Riedel authored Nov 19, 2015 and Ben Gamari committed Nov 19, 2015

`derivedConstants` currently uses `System.Info.os` for decisions (which
doesn't necessarily reflect the build-target), as well as hardcoding
"/usr/bin/objdump" for openbsd.

This patch auto-detects `objdump` similiar to how `nm` is detected via
Autoconf as well as passing the target-os into `derivedConstants` via
commandline.

Reviewers: austin, kgardas, erikd, bgamari

Reviewed By: kgardas, erikd, bgamari

Subscribers: kgardas, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1499