wgengine/netstack/gro: permit building without GRO

This only saves ~32KB in the minimal linux/amd64 binary, but it's a
step towards permitting not depending on gvisor for small builds.

Updates #17283

Change-Id: Iae8da5e9465127de354dbcaf25e794a6832d891b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_gro_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_gro
+
+package buildfeatures
+
+// HasGRO is whether the binary was built with support for modular feature "Generic Receive Offload support (performance)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_gro" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasGRO = false

feature/buildfeatures/feature_gro_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_gro
+
+package buildfeatures
+
+// HasGRO is whether the binary was built with support for modular feature "Generic Receive Offload support (performance)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_gro" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasGRO = true