This reverts commit d6cecde5.

The patch broke Simon PJ's Windows build, becuase he didn't
have (and should not need) a separate msys2 gcc.

Following an exchange on the ghc-devs list, Tamar wrote

  Oops, sorry, didn’t notice it because both mine and harbormaster’s
  msys2 have separate GCCs installed as well.

  I don’t see an easy fix that would also work for end user Configure
  based cabal installs. So I think I’ll have to go back to the drawing
  board for this one.

  You can just leave it reverted.