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.go

@@ -33,6 +33,6 @@ func DefaultSpec() specs.Platform {
 		OS:           runtime.GOOS,
 		Architecture: runtime.GOARCH,
 		// The Variant field will be empty if arch != ARM.
-		Variant: cpuVariant,
+		Variant: cpuVariant(),
 	}
 }