misc/dist: build race packages when os suffix present

The "darwin-amd64-osx10.8" target was not matching "darwin-amd64".

R=golang-dev
CC=golang-dev
https://golang.org/cl/14930043

misc/dist/bindist.go

@@ -175,7 +175,7 @@ func main() {
 			}
 			if *includeRace {
 				for _, t := range raceAvailable {
-					if t == targ {
+					if t == targ || strings.HasPrefix(targ, t+"-") {
 						b.Race = true
 					}
 				}