cmd/internal/obj: remove Link.Windows field

This CL addresses a long standing CL by rsc by pushing the use of
Link.Windows down to its two users.

Link.Window was always initalised with the value of runtime.GOOS so
this does not affect cross compilation.

Change-Id: Ibbae068f8b5aad06336909691f094384caf12352
Reviewed-on: https://go-review.googlesource.com/20869
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -16,6 +16,7 @@ import (
 	"log"
 	"os"
 	"path"
+	"runtime"
 	"strconv"
 	"strings"
 )
@@ -571,7 +572,7 @@ func isDriveLetter(b byte) bool {
 // is this path a local name?  begins with ./ or ../ or /
 func islocalname(name string) bool {
 	return strings.HasPrefix(name, "/") ||
-		Ctxt.Windows != 0 && len(name) >= 3 && isDriveLetter(name[0]) && name[1] == ':' && name[2] == '/' ||
+		runtime.GOOS == "windows" && len(name) >= 3 && isDriveLetter(name[0]) && name[1] == ':' && name[2] == '/' ||
 		strings.HasPrefix(name, "./") || name == "." ||
 		strings.HasPrefix(name, "../") || name == ".."
 }
@@ -893,7 +894,7 @@ func mkpackage(pkgname string) {
 		if i := strings.LastIndex(p, "/"); i >= 0 {
 			p = p[i+1:]
 		}
-		if Ctxt.Windows != 0 {
+		if runtime.GOOS == "windows" {
 			if i := strings.LastIndex(p, `\`); i >= 0 {
 				p = p[i+1:]
 			}

src/cmd/internal/obj/link.go

@@ -618,7 +618,6 @@ type Link struct {
 	Flag_optimize bool
 	Bso           *Biobuf
 	Pathname      string
-	Windows       int32
 	Goroot        string
 	Goroot_final  string
 	Hash          map[SymVer]*LSym

src/cmd/internal/obj/sym.go

@@ -35,7 +35,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strconv"
 )
 
@@ -83,10 +82,6 @@ func Linknew(arch *LinkArch) *Link {
 	ctxt.Version = HistVersion
 	ctxt.Goroot = Getgoroot()
 	ctxt.Goroot_final = os.Getenv("GOROOT_FINAL")
-	if runtime.GOOS == "windows" {
-		// TODO(rsc): Remove ctxt.Windows and let callers use runtime.GOOS.
-		ctxt.Windows = 1
-	}
 
 	var buf string
 	buf, _ = os.Getwd()