switch to ASM vector checksum

overlay/checksum/checksum_arm64.go

@@ -0,0 +1,12 @@
+package checksum
+
+//go:noescape
+func checksumNEON(buf []byte, initial uint16) uint16
+
+// Checksum computes the RFC 1071 ones-complement sum of buf, seeded with
+// initial. It is a drop-in replacement for gvisor's checksum.Checksum
+// that dispatches to a hand-written NEON routine. NEON is mandatory in
+// armv8 so no feature check is needed.
+func Checksum(buf []byte, initial uint16) uint16 {
+	return checksumNEON(buf, initial)
+}