Summary:
Systools makes some pretty hard assumptions about where GHC is on Windows.
One of these is that ghc be in a folder named `bin` and that `../lib` exists.

This pattern doesn't hold for symlinks as a link `C:\ghc-bin\`
pointing to `C:\ghc\ghc-7.10.3\bin` will break this assumption.

This patch resolves symlinks by finding where they point to and uses that location
as the base for GHC.

This uses an API that's been introduced in Vista. For older systems it falls back to
the current behavior of not resolving symlinks.

Test Plan:
1) Create symlink to GHC's bin folder.
2) Run GHC from that folder.

Reviewers: austin, bgamari

Reviewed By: austin

Subscribers: #ghc_windows_task_force, thomie

Differential Revision: https://phabricator.haskell.org/D2101

GHC Trac Issues: #11759