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/10849
Reviewed-by: Minux Ma <minux@golang.org>
diff --git a/src/cmd/compile/internal/gc/mkbuiltin.go b/src/cmd/compile/internal/gc/mkbuiltin.go
index f4569b4..ea3877f 100644
--- a/src/cmd/compile/internal/gc/mkbuiltin.go
+++ b/src/cmd/compile/internal/gc/mkbuiltin.go
@@ -46,7 +46,7 @@
 	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)