ghc.mk: fix 'make install' for cross-mingw32

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>

ghc.mk

@@ -924,7 +924,7 @@ ifneq "$(INSTALL_LIBEXECS)" ""
 	done
 # We rename ghc-stage2, so that the right program name is used in error
 # messages etc.
-ifeq "$(Windows_Host)" "NO"
+ifeq "$(Windows_Target)" "NO"
 	"$(MV)" "$(DESTDIR)$(ghclibexecdir)/bin/ghc-stage$(INSTALL_GHC_STAGE)" "$(DESTDIR)$(ghclibexecdir)/bin/ghc"
 endif
 endif