Support thin archive format

This is a patch from FB's internal build of GHC that I'm pushing
upstream.

Author: Andrew Gallagher <agallagher@fb.com>

This diff adds simple thin archive support to ghc's linker code, which
basically just entails finding the member data from disk rather than
from inside the archive (except for the case of the symbol index and
gnu filename index, where the member data is still inline).