Add tailscale checksum

internal/tschecksum/checksum_generic.go

@@ -0,0 +1,15 @@
+// This file contains IP checksum algorithms that are not specific to any
+// architecture and don't use hardware acceleration.
+
+//go:build !amd64
+
+package tschecksum
+
+import "strconv"
+
+func Checksum(data []byte, initial uint16) uint16 {
+	if strconv.IntSize < 64 {
+		return checksumGeneric32(data, initial)
+	}
+	return checksumGeneric64(data, initial)
+}