Summary:
GHC's configure script seems to normalize the values returned from config.guess.
So for Windows it turns x86_64-pc-mingw64 into x86_64-unknown-mingw32.
These mangled names are stored in the values $BuildPlatform, $HostPlatform
and $TargetPlatform.

However further down the file when the comparison is done between the stage0
compiler and the host the normalized versions are not used.
So when normalization actually changes the triple this check will fail.

Not sure why it's worked for all this time.. Nor if this is the right fix?
Does it still work for cross compiling correctly?

Test Plan: ./configure

Reviewers: hvr, austin, thomie, bgamari, erikd

Reviewed By: erikd

Subscribers: erikd, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2452

GHC Trac Issues: #12487