Index: go/src/go/build/build.go
===================================================================
--- go.orig/src/go/build/build.go
+++ go/src/go/build/build.go
@@ -1388,7 +1388,7 @@ func init() {
 }
 
 // ToolDir is the directory containing build tools.
-var ToolDir = filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+var ToolDir = envOr("GOTOOLDIR", filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH))
 
 // IsLocalImport reports whether the import path is
 // a local import path, like ".", "..", "./foo", or "../foo".
Index: go/src/cmd/go/build.go
===================================================================
--- go.orig/src/cmd/go/build.go
+++ go/src/cmd/go/build.go
@@ -1312,7 +1312,7 @@ func (b *builder) build(a *action) (err
 		}
 
 		cgoExe := tool("cgo")
-		if a.cgo != nil && a.cgo.target != "" {
+		if a.cgo != nil && a.cgo.target != "" && os.Getenv("GOTOOLDIR") == "" {
 			cgoExe = a.cgo.target
 		}
 		outGo, outObj, err := b.cgo(a.p, cgoExe, obj, pcCFLAGS, pcLDFLAGS, cgofiles, gccfiles, cxxfiles, a.p.MFiles)