cmd/buildlet: add freebsd-riscv64
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 <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

golang/build,golang/build,golang/build,golang/build