unicode/maketables: add -output flag, buffer output, use gofmt

Simplify the invocation (and speed it up substantially) in preparation
for move to go generate.

LGTM=bradfitz, mpvl
R=mpvl, bradfitz, josharian
CC=golang-codereviews
https://golang.org/cl/135790043

src/pkg/unicode/Makefile

@@ -6,8 +6,7 @@ maketables: maketables.go
 	go build maketables.go
 
 tables:	maketables
-	./maketables --tables=all > tables.go
-	gofmt -w tables.go
+	./maketables --tables=all -output tables.go
 
 # Downloads from www.unicode.org, so not part
 # of standard test scripts.