fix 1.9.4 cross compile problem

gc.go

@@ -126,6 +126,12 @@ func (t *gcToolchain) Gc(pkg *Package, files []string) error {
 	for _, d := range pkg.includePaths() {
 		args = append(args, "-I", d)
 	}
+
+	if (version.Version > 1.8) && pkg.Goroot {
+		// stdlib with special std flag
+		args = append(args, "-std")
+	}
+
 	if pkg.Goroot && pkg.ImportPath == "runtime" {
 		// runtime compiles with a special gc flag to emit
 		// additional reflect type data.

internal/version/version16.go

@@ -1,5 +1,5 @@
 // +build go1.6
-// +build !go1.7,!go1.8
+// +build !go1.7,!go1.8,!go1.9,!go1.10
 
 package version
 

internal/version/version17.go

@@ -1,5 +1,5 @@
 // +build go1.7
-// +build !go1.8
+// +build !go1.8,!go1.9,!go1.10
 
 package version
 

internal/version/version18.go

@@ -1,4 +1,5 @@
 // +build go1.8
+// +build !go1.9,!go1.10
 
 package version
 

internal/version/version19.go

@@ -0,0 +1,7 @@
+// +build go1.9,go1.10
+
+package version
+
+const (
+	Version = 1.9
+)