Add tailscale checksum

internal/gtcpip/checksum/checksum_default.go

@@ -0,0 +1,13 @@
+//go:build !amd64
+
+package checksum
+
+// Checksum calculates the checksum (as defined in RFC 1071) of the bytes in the
+// given byte array. This function uses an optimized version of the checksum
+// algorithm.
+//
+// The initial checksum must have been computed on an even number of bytes.
+func Checksum(buf []byte, initial uint16) uint16 {
+	s, _ := calculateChecksum(buf, false, initial)
+	return s
+}