cmd/internal/gc: mv builtins builtin

This directory is processed by mkbuiltin.go and generates builtin.go.
It should be named builtin too, not builtins, both for consistency
and because file and directory names in general are singular unless
forced otherwise.

Commented on CL 6233 too.

Change-Id: Ic5d3671443ae9292b69fda118f61a11c88d823fa
Reviewed-on: https://go-review.googlesource.com/7660Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/internal/gc/mkbuiltin.go

@@ -50,7 +50,7 @@ func main() {
 
 // Compile .go file, import data from .6 file, and write Go string version.
 func mkbuiltin(w io.Writer, gochar string, name string) {
-	if err := exec.Command("go", "tool", gochar+"g", "-A", "builtins/"+name+".go").Run(); err != nil {
+	if err := exec.Command("go", "tool", gochar+"g", "-A", "builtin/"+name+".go").Run(); err != nil {
 		log.Fatal(err)
 	}
 	obj := fmt.Sprintf("%s.%s", name, gochar)