ghc.mk: rename installed ghc-stage1 on non-windows

When user installs _native_ build ghc executable is renamed
from '$(libexec)/bin/ghc-stage<N>' to '$(libexec)/bin/ghc'.
But not on windows!

In case of _cross-compiler_ rename should happen only
for '$(libexec)/bin/ghc-stage<N>' runnable on non-windows
platform.

Before the change '$(libexec)/bin/ghc-stage<N>' rename happened
for any compiler not targeting windows.

After the patch rename also happens for '$(libexec)/bin/ghc-stage1'
cross-compiler built for linux targeting windows (Stage1Only=YES case).

Or on a concrete example:

   # host is x86_64-pc-linux-gnu
   $ ./configure --target=i686-w64-mingw32
   $ make install Stage1Only=YES

Before the change the layout was:
   - '$(libexec)/bin/ghc-stage1' was installed
   - bin/ghc contained 'exec $(libexec)/bin/ghc' # missing file!
After the change:
   - '$(libexec)/bin/ghc' was installed
   - bin/ghc contained 'exec $(libexec)/bin/ghc' # present file

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