See `Note [Replacing backward slashes in config.libdir]`

There is one caveat: in ae4acbd1
I mentioned:

  > Changing backwards slashes to forward slashes apparently confuses
  > msys2/mingw magic path handling.

I can not reproduce that problem anymore, however.

This patch validates for me, and fixes all tests that use config.libdir
for WAY=ghci. We'll see how it goes.