Automatic merge from submit-queue

Automatically add node labels beta.kubernetes.io/{os,arch}

Proposal: #17981
As discussed in #22623:
> @davidopp: #9044 says cloud provider but can also cover platform stuff.

Adds a label `beta.kubernetes.io/platform` to `kubelet` that informs about the os/arch it's running on.
Makes it easy to specify `nodeSelectors` for different arches in multi-arch clusters.

```console
$ kubectl get no --show-labels
NAME        STATUS    AGE       LABELS
127.0.0.1   Ready     1m        beta.kubernetes.io/platform=linux-amd64,kubernetes.io/hostname=127.0.0.1
$ kubectl describe no
Name:			127.0.0.1
Labels:			beta.kubernetes.io/platform=linux-amd64,kubernetes.io/hostname=127.0.0.1
CreationTimestamp:	Thu, 31 Mar 2016 20:39:15 +0300
```
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