build: add missing nuke target

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5266046

src/cmd/Makefile

@@ -48,6 +48,7 @@ CLEANDIRS=\
 
 install: $(patsubst %,%.install,$(DIRS))
 clean: $(patsubst %,%.clean,$(CLEANDIRS))
+nuke: $(patsubst %,%.nuke,$(CLEANDIRS))
 
 %.install:
 	@echo
@@ -62,5 +63,8 @@ $(O)a.install $(O)c.install $(O)g.install: $(O)l.install
 %.clean:
 	$(MAKE) -C $* clean
 
+%.nuke:
+	$(MAKE) -C $* nuke
+
 echo-dirs:
 	@echo $(DIRS)