Remove the Windows GCC driver.

Summary:
This patch drops the GCC driver and instead moves
the only remaining path that we need to keep for
backwards compatibility to the settings file.

It also generalizes the code that expands `$TopDir`
so it can expand it within any location in the string
and also changes it so `$TopDir` is expanded only
after the words call because `$TopDir` can contains
spaces which would be horribly broken.

Test Plan: ./validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

GHC Trac Issues: #13709

Differential Revision: https://phabricator.haskell.org/D3592