race.bash: support freebsd

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/110150044

src/race.bash

@@ -9,7 +9,7 @@
 set -e
 
 function usage {
-	echo 'race detector is only supported on linux/amd64 and darwin/amd64' 1>&2
+	echo 'race detector is only supported on linux/amd64, freebsd/amd64 and darwin/amd64' 1>&2
 	exit 1
 }
 
@@ -25,6 +25,11 @@ case $(uname) in
 		usage
 	fi
 	;;
+"FreeBSD")
+	if [ $(uname -m) != "amd64" ]; then
+		usage
+	fi
+	;;
 *)
 	usage
 	;;