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 <Wei.Chen@arm.com>
Signed-off-by: Penny Zheng <Penny.Zheng@arm.com>

platforms/defaults.go

@@ -16,6 +16,7 @@ func DefaultSpec() specs.Platform {
 	return specs.Platform{
 		OS:           runtime.GOOS,
 		Architecture: runtime.GOARCH,
-		// TODO(stevvooe): Need to resolve GOARM for arm hosts.
+		// The Variant field will be empty if arch != ARM.
+		Variant: cpuVariant,
 	}
 }