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>