The build failure when cross-compiling from
linux to windows looks like:

    HC [stage 1] ghc/stage2/build/tmp/ghc-stage2.exe
      Call hs_init_ghc() from your main() function to set these options.
  /usr/libexec/gcc/i686-w64-mingw32/ld: cannot find -lHSghc-8.3-0

Similar to commit 745032dd


("rts: tweak cross-compilation to mingw32") decision to split
stage2 should be done based based on TargetOS, not HostOS.

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