Derive cpuinfo as needed, instead of at init-time

This changes platforms.Parse to hit /proc to look up CPU info only when
it's needed, instead of in init(). This makes the package a bit easier
for other packages to consume, especially clients that don't call
platforms.Parse or need to lookup CPU info.

Signed-off-by: Jason Hall <jasonhall@redhat.com>

platforms/defaults_test.go

@@ -28,7 +28,7 @@ func TestDefault(t *testing.T) {
 	expected := specs.Platform{
 		OS:           runtime.GOOS,
 		Architecture: runtime.GOARCH,
-		Variant:      cpuVariant,
+		Variant:      cpuVariant(),
 	}
 	p := DefaultSpec()
 	if !reflect.DeepEqual(p, expected) {