ctr: Add variant to ARM's DefaultSpec string
ctr: Add variant to ARM's DefaultSpec string

The variant is required for platform match while pulling images
for ARM platforms. Currently, the cpuVariant only would be assigned
on linux/arm|arm64 platforms. Other platforms this variable would
be empty. So we can use this cpuVariant to initialize the Variant
field.

Change-Id: Ic065be9b502f1e662445daa61a0973bf56385b37
Signed-off-by: Wei Chen <7bda119aa2bd1d28c0fb067e862b11d5acc5fc93@arm.com>
Signed-off-by: Penny Zheng <88ce651a49bdc66ed2a0360c00692a1df24dd142@arm.com>

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