ghc.mk: Use Windows_Target instead of Windows_Host
ghc.mk: Use Windows_Target instead of Windows_Host

This is a step towards building a Linux to Windows cross-compiler.

Test Plan: Build on Linux and Windows

Reviewers: bgamari, hvr, austin, Phyx

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1758

GHC Trac Issues: #10070

vikraman/ghc,oldmanmike/ghc,tjakway/ghcjvm,snoyberg/ghc,tjakway/ghcjvm,sdiehl/ghc,nushio3/ghc,snoyberg/ghc,snoyberg/ghc,sgillespie/ghc,nushio3/ghc,shlevy/ghc,gridaphobe/ghc,tjakway/ghcjvm,mettekou/ghc,shlevy/ghc,sgillespie/ghc,mcschroeder/ghc,mcschroeder/ghc,oldmanmike/ghc,ezyang/ghc,mettekou/ghc,nushio3/ghc,oldmanmike/ghc,olsner/ghc,ezyang/ghc,vikraman/ghc,vikraman/ghc,sgillespie/ghc,gridaphobe/ghc,vTurbine/ghc,snoyberg/ghc,vikraman/ghc,sdiehl/ghc,ezyang/ghc,nushio3/ghc,shlevy/ghc,vTurbine/ghc,gridaphobe/ghc,vikraman/ghc,olsner/ghc,olsner/ghc,tjakway/ghcjvm,ezyang/ghc,sgillespie/ghc,mcschroeder/ghc,olsner/ghc,vTurbine/ghc,vikraman/ghc,sgillespie/ghc,nushio3/ghc,mcschroeder/ghc,oldmanmike/ghc,vTurbine/ghc,vTurbine/ghc,oldmanmike/ghc,oldmanmike/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,mettekou/ghc,snoyberg/ghc,tjakway/ghcjvm,olsner/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,mcschroeder/ghc,gridaphobe/ghc,ezyang/ghc,tjakway/ghcjvm,sgillespie/ghc,sdiehl/ghc,vTurbine/ghc,vTurbine/ghc,shlevy/ghc,ezyang/ghc,vikraman/ghc,nushio3/ghc,sgillespie/ghc,shlevy/ghc,oldmanmike/ghc,nushio3/ghc,sdiehl/ghc,tjakway/ghcjvm,mettekou/ghc,mcschroeder/ghc,sdiehl/ghc,shlevy/ghc,gridaphobe/ghc,olsner/ghc,gridaphobe/ghc,sdiehl/ghc,gridaphobe/ghc,olsner/ghc,mettekou/ghc,mcschroeder/ghc,snoyberg/ghc