cmd/dist: emit \r in env -w output

go tool dist env -w is supposed to print a Windows batch file.
Normally Windows will execute batch files without \r before \n,
but issue 3060 reports that if the file ends up containing paths
written in Chinese, Windows 7 cannot execute it without the \r.
So add the \r.

Fixes #3060.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6498120

src/cmd/dist/build.c

@@ -1352,7 +1352,7 @@ cmdenv(int argc, char **argv)
 		vflag++;
 		break;
 	case 'w':
-		format = "set %s=%s\n";
+		format = "set %s=%s\r\n";
 		break;
 	default:
 		usage();