Simon Marlow authored Oct 14, 2016 and Ben Gamari committed Oct 14, 2016

This enables loading dynamic libraries that refer to the RTS.  I just
came across somewhere I needed to do that, and without
`--export-dynamic` it's impossible.

For now we'll only support that when using `-fexternal-interpreter`,
because the dynamic symbol table for GHC itself is much bigger.

Test Plan: validate

Reviewers: niteria, austin, erikd, bgamari

Reviewed By: bgamari

Subscribers: Phyx, thomie

Differential Revision: https://phabricator.haskell.org/D2590