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/platforms.go

@@ -189,8 +189,8 @@ func Parse(specifier string) (specs.Platform, error) {
 		if isKnownOS(p.OS) {
 			// picks a default architecture
 			p.Architecture = runtime.GOARCH
-			if p.Architecture == "arm" && cpuVariant != "v7" {
-				p.Variant = cpuVariant
+			if p.Architecture == "arm" && cpuVariant() != "v7" {
+				p.Variant = cpuVariant()
 			}
 
 			return p, nil