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_test.go

@@ -31,8 +31,8 @@ func TestParseSelector(t *testing.T) {
 		defaultVariant = ""
 	)
 
-	if defaultArch == "arm" && cpuVariant != "v7" {
-		defaultVariant = cpuVariant
+	if defaultArch == "arm" && cpuVariant() != "v7" {
+		defaultVariant = cpuVariant()
 	}
 
 	for _, testcase := range []struct {