ctr: Identify the platform.variant for ARM platforms
ctr: Identify the platform.variant for ARM platforms

In the commit "26329b2b8d7fd4e290b2b0f0163547f2d79bb817",
dmcgowan/fix-pull-multi-arch, PR#1535. The containerd has enabled the pull
multi-arch image support. But the platform.variant field of OCI for ARM
hadn't been ready at that time, so all ARM images could not pull successfully.

"
docker.io/library/hello-world:latest:
resolved       |++++++++++++++++++++++++++++++++++++++|
index-sha256:3644c0788e3d3823f9e97f757f01d2ddc6eb5458df9d801:
done           |++++++++++++++++++++++++++++++++++++++|
elapsed: 5.1 s
total:  2.7 Ki (533.0 B/s)
unpacking sha256:3d3823f9e97f757f01d2ddc6eb5458df9d801...
ctr: : manifest not found: not found
"

In this patch we'll detect the ARM variants from /proc/cpuinfo. Because Linux
kernel has already detected the ABI, ISA and Features for us. We don't need to
parse them from registers again.

Change-Id: I479b34bf3f52df9f7a6b3c77718b7d316dbf7f69
Signed-off-by: Wei Chen <7bda119aa2bd1d28c0fb067e862b11d5acc5fc93@arm.com>
Signed-off-by: Penny Zheng <88ce651a49bdc66ed2a0360c00692a1df24dd142@arm.com>

estesp/containerd,mikebrow/containerd,kunalkushwaha/containerd,crosbymichael/containerd,thaJeztah/containerd,kunalkushwaha/containerd,vdemeester/containerd,dmcgowan/containerd,thaJeztah/containerd,containerd/containerd,vdemeester/containerd,dmcgowan/containerd,crosbymichael/containerd,containerd/containerd,miaoyq/containerd,mikebrow/containerd,estesp/containerd,miaoyq/containerd