cmd/dist: fix build for Linux/ARM

        find() returns -1 when not found.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5752068

src/cmd/dist/build.c

@@ -1321,7 +1321,7 @@ cmdenv(int argc, char **argv)
 	xprintf(format, "GOTOOLDIR", tooldir);
 	xprintf(format, "GOCHAR", gochar);
 
-	if(find(bprintf(&b, "%s/%s", goos, goarch), okcgo, nelem(okcgo)))
+	if(find(bprintf(&b, "%s/%s", goos, goarch), okcgo, nelem(okcgo)) >= 0)
 		xprintf(format, "CGO_ENABLED", "1");
 	else
 		xprintf(format, "CGO_ENABLED", "0");