build: allow clean.bash to work on fresh checkout

Must be invoked as ./clean.bash --gomake make
(or --gomake gmake, depending on the name of
GNU make).

R=niemeyer
CC=golang-dev
https://golang.org/cl/4023065

src/clean.bash

@@ -25,8 +25,8 @@ for i in lib9 libbio libmach cmd pkg \
 do(
 	cd "$GOROOT"/src/$i || exit 1
 	if test -f clean.bash; then
-		bash clean.bash
+		bash clean.bash --gomake $MAKE
 	else
-		gomake clean
+		$MAKE clean
 	fi
 )done