build: use correct go-tool directory location

R=golang-dev, rsc, cw, rsc
CC=golang-dev
https://golang.org/cl/5576070

src/cmd/cov/Makefile

@@ -38,4 +38,4 @@ install-darwin: $(TARG)
 	@true
 
 install-default: $(TARG)
-	cp $(TARG) "$(GOBIN)"/go-tool/$(TARG)
+	cp $(TARG) "$(GOROOT)"/bin/go-tool/$(TARG)

src/cmd/go/tool.go

@@ -9,6 +9,7 @@ import (
 	"go/build"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"sort"
 	"strings"
 )
@@ -28,7 +29,7 @@ For more about each tool command, see 'go tool command -h'.
 var (
 	toolGoos       = build.DefaultContext.GOOS
 	toolIsWindows  = toolGoos == "windows"
-	toolBinToolDir = build.Path[0].BinDir() + "/go-tool"
+	toolBinToolDir = filepath.Join(build.Path[0].Path, "bin", "go-tool")
 )
 
 const toolWindowsExtension = ".exe"

src/cmd/prof/Makefile

@@ -32,7 +32,7 @@ install-darwin: $(TARG)
 	@true
 
 install-default: $(TARG)
-	cp $(TARG) "$(GOBIN)"/go-tool/$(TARG)
+	cp $(TARG) "$(GOROOT)"/bin/go-tool/$(TARG)
 
 install-pprof: pprof
-	cp pprof "$(GOBIN)"/go-tool/pprof
+	cp pprof "$(GOROOT)"/bin/go-tool/pprof