mkbuiltin: generate builtin.c directly

R=golang-dev, rsc
https://golang.org/cl/173041

src/cmd/gc/Makefile

@@ -55,7 +55,7 @@ y.tab.c: y.tab.h
 	test -f y.tab.c && touch y.tab.c
 
 builtin.c:	runtime.go unsafe.go mkbuiltin1.c mkbuiltin
-	./mkbuiltin >builtin.c || \
+	./mkbuiltin || \
 	(echo 'mkbuiltin failed; using bootstrap copy of builtin.c'; cp builtin.c.boot builtin.c)
 
 subr.$O: opnames.h

src/cmd/gc/mkbuiltin

@@ -35,6 +35,4 @@ ken | r | rsc)
 	fi
 esac
 
-cat _builtin.c
-rm -f _builtin.c
-
+mv _builtin.c builtin.c