Add cpu value riscv64

cpu/BUILD

diff --git a/cpu/BUILD b/cpu/BUILD
index 00c2901..ee2e8e8 100644
--- a/cpu/BUILD
+++ b/cpu/BUILD
@@ -119,3 +119,9 @@
     name = "mips64",
     constraint_setting = ":cpu",
 )
+
+constraint_value(
+    name = "riscv64",
+    constraint_setting = ":cpu",
+)
+