ghc, ghc-pkg: use getExecutablePath on Windows when base >= 4.11.0

Authored by Phyx on Aug 11 2018, 1:25 PM.

Description

ghc, ghc-pkg: use getExecutablePath on Windows when base >= 4.11.0

Summary:
This completes the work started in D4227 by using just 'getExecutablePath'
in ghc and ghc-pkg when building with base >= 4.11.0.

On the long term, we will be able to simply kill the existing code that
follows (or not) symlinks and just get this behaviour for free from
getExecutable. For now we however have to require base >= 4.11.0 to be able
to just use getExecutablePath under Windows, and use the current code when
building with an older base.

Original code by @alpmestan commandeering since patch has been stale
and bug remains open.

Test Plan: Validate

Reviewers: angerman, bgamari, erikd, alpmestan

Reviewed By: bgamari

Subscribers: carter, rwbarton, thomie

GHC Trac Issues: Trac #14483

Differential Revision: https://phabricator.haskell.org/D4229