Summary:
Code already existed in the RTS to add thin library support for non-Windows
operating systems. This adds it to Windows as well.

ar thin libraries have the exact same format as normal archives except they
have a different magic string and they don't copy the object files into the
archive.

Instead each header entry points to the location of the object file on disk.
This is useful when a library is only created to satisfy a compile time dependency
instead of to be distributed. This saves the time required for copying.

Test Plan: ./validate and new test T11788

Reviewers: austin, bgamari, simonmar, erikd

Reviewed By: bgamari, simonmar

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2323

GHC Trac Issues: #11788