cpu: add cpu_freebsd_riscv64.go

Copied from the arm64 definitions.

cpu/cpu_freebsd_riscv64.go

@@ -0,0 +1,9 @@
+package cpu
+
+type cpuTimes struct {
+	User uint64
+	Nice uint64
+	Sys  uint64
+	Intr uint64
+	Idle uint64
+}