crypto: update go.mod for s390x x/sys/cpu changes

To fix broken builds like:
https://build.golang.org/log/4c3d8463f5c2b2478dc2315f6c685b2f77bfcdd1

Change-Id: I4c4866e5928a73856ca8a411ed97588e6f66e490
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/170623
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/go.mod b/go.mod
index 3a8a43e..d2b06ca 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module golang.org/x/crypto
 
-require golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a
+require golang.org/x/sys v0.0.0-20190403152447-81d4e9dc473e
diff --git a/go.sum b/go.sum
index 1936851..be20559 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,2 @@
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a h1:1BGLXjeY4akVXGgbC9HugT3Jv3hCI0z56oJR5vAMgBU=
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190403152447-81d4e9dc473e h1:nFYrTHrdrAOpShe27kaFHjsqYSEQ0KWqdWLu3xuZJts=
+golang.org/x/sys v0.0.0-20190403152447-81d4e9dc473e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=