runtime: add cgo call benchmark

Change-Id: I12d2ae7dd6a33ecb7110b7d090871e7143fd609f
Reviewed-on: https://go-review.googlesource.com/c/go/+/646196
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/runtime/cgobench/funcs.go

@@ -0,0 +1,17 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cgo
+
+package cgobench
+
+/*
+static void empty() {
+}
+*/
+import "C"
+
+func Empty() {
+	C.empty()
+}