cmd/go: say that -race flag can be used on windows/amd64

R=golang-dev, r
CC=dvyukov, golang-dev
https://golang.org/cl/6810099

src/cmd/go/build.go

@@ -62,7 +62,8 @@ The build flags are shared by the build, install, run, and test commands:
 		print the commands.
 	-race
 		enable data race detection.
-		Currently supported only on linux/amd64 and darwin/amd64.
+		Currently supported only on linux/amd64,
+		darwin/amd64 and windows/amd64.
 
 	-ccflags 'arg list'
 		arguments to pass on each 5c, 6c, or 8c compiler invocation