Add tailscale checksum

internal/tschecksum/checksum_amd64.go

@@ -0,0 +1,23 @@
+package tschecksum
+
+import "golang.org/x/sys/cpu"
+
+var checksum = checksumAMD64
+
+// Checksum computes an IP checksum starting with the provided initial value.
+// The length of data should be at least 128 bytes for best performance. Smaller
+// buffers will still compute a correct result.
+func Checksum(data []byte, initial uint16) uint16 {
+	return checksum(data, initial)
+}
+
+func init() {
+	if cpu.X86.HasAVX && cpu.X86.HasAVX2 && cpu.X86.HasBMI2 {
+		checksum = checksumAVX2
+		return
+	}
+	if cpu.X86.HasSSE2 {
+		checksum = checksumSSE2
+		return
+	}
+}