cmd/compile: fix builtin generation

This was a refactoring bug during
'go tool compile', CL 10289.

Change-Id: Ibfd333be39ec72bba331fdf352df619cc21851a9
Reviewed-on: https://go-review.googlesource.com/10849Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -46,7 +46,7 @@ func mkbuiltin(w io.Writer, name string) {
 	if err := exec.Command("go", "tool", "compile", "-A", "builtin/"+name+".go").Run(); err != nil {
 		log.Fatal(err)
 	}
-	obj := "name.o"
+	obj := name + ".o"
 	defer os.Remove(obj)
 
 	r, err := os.Open(obj)