Attempt to install cross-compiled mingw32 GHC built on linux failed as:

    $ make install DESTDIR=$(pwd)/__i__

    "mv" "$(pwd)/__i__/usr/local/lib/ghc-8.3.20170422/bin/ghc-stage2" \
         "$(pwd)/__i__/usr/local/lib/ghc-8.3.20170422/bin/ghc"
    mv: failed to stat
         '$(pwd)/__i__/usr/local/lib/ghc-8.3.20170422/bin/ghc-stage2': \
         No such file or directory

The rename should not be performed for windows targets.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>