Update poly1305 upstream library with ARM-related fixes

Signed-off-by: Sergey Matveev <stargrave@stargrave.org>

diff --git a/src/golang.org/x/crypto b/src/golang.org/x/crypto
index 2f3083f6163ef51179ad42ed523a18c9a1141467..81bf7719a6b7ce9b665598222362b50122dfc13b 160000 (submodule)
--- a/src/golang.org/x/crypto
+++ b/src/golang.org/x/crypto
@@ -1 +1 @@
-Subproject commit 2f3083f6163ef51179ad42ed523a18c9a1141467
+Subproject commit 81bf7719a6b7ce9b665598222362b50122dfc13b