Use uname machine field to get CPU variant if fails at /proc/cpuinfo

When images/containers in ARM arch were built/executed on x86 host,
getCPUVariant will fail as it tries to look for /proc/cpuinfo, whose
content is from the host. Adding a new method as fallback to check uname
machine when it happens.

Signed-off-by: Tony Fang <nenghui.fang@gmail.com>

platforms/cpuinfo_linux_test.go

@@ -0,0 +1,53 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package platforms
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestCPUVariant(t *testing.T) {
+	if !isArmArch(runtime.GOARCH) {
+		t.Skip("only relevant on linux/arm")
+	}
+
+	variants := []string{"v8", "v7", "v6", "v5", "v4", "v3"}
+
+	p, err := getCPUVariant()
+	if err != nil {
+		t.Fatalf("Error getting CPU variant: %v\n", err)
+		return
+	}
+
+	for _, variant := range variants {
+		if p == variant {
+			t.Logf("got valid variant as expected: %#v = %#v\n", p, variant)
+			return
+		}
+	}
+
+	t.Fatalf("could not get valid variant as expected: %v\n", variants)
+}
+
+func TestGetCPUVariantFromArch(t *testing.T) {
+
+	if !isArmArch(runtime.GOARCH) {
+		t.Skip("only relevant on linux/arm")
+	}
+
+}