cmd/buildlet: add freebsd-riscv64

For golang/go#53466

Change-Id: Ibe1355980422cd9c4ddb1029b09046f8bcc41a5a
Reviewed-on: https://go-review.googlesource.com/c/build/+/432658
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/cmd/buildlet/Makefile b/cmd/buildlet/Makefile
index 002bee6..3ed15e4 100644
--- a/cmd/buildlet/Makefile
+++ b/cmd/buildlet/Makefile
@@ -22,6 +22,7 @@
 	buildlet.freebsd-arm \
 	buildlet.freebsd-arm-5 \
 	buildlet.freebsd-arm-7 \
+	buildlet.freebsd-riscv64 \
 	buildlet.illumos-amd64 \
 	buildlet.linux-amd64 \
 	buildlet.linux-arm \