go.mod: update x/sys to master

run:
  $ go get -u -d golang.org/x/sys@master
  $ go mod tidy

Change-Id: If8eaa7ec31971bca33155f3ed04db0ab26cf7697
Reviewed-on: https://go-review.googlesource.com/c/debug/+/207197
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>