std,cmd: sync go.mod with new release branches
    
        cd src
        go get golang.org/x/sys@release-branch.go1.14-std
        go mod tidy && go mod vendor
        cd cmd
        go get golang.org/x/sys@release-branch.go1.14-cmd
        go get golang.org/x/tools@release-branch.go1.14
        go mod tidy && go mod vendor
    
    Fixes #36851
    
    Change-Id: Ib0a78eec90c92515b6f421972102cc8e8d0c08a7
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217305
    Run-TryBot: Filippo Valsorda <filippo@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>