Add CPU level test (#96)

Example: `cpuid -check-level=4`

cmd/cpuid/main.go

@@ -14,16 +14,30 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"log"
 	"os"
 	"strings"
 
 	"github.com/klauspost/cpuid/v2"
 )
 
 var js = flag.Bool("json", false, "Output as JSON")
+var level = flag.Int("check-level", 0, "Check microarchitecture level. Exit code will be 0 if supported")
 
 func main() {
 	flag.Parse()
+	if level != nil && *level > 0 {
+		if *level < 1 || *level > 4 {
+			log.Fatalln("Supply CPU level 1-4 to test as argument")
+		}
+		log.Println(cpuid.CPU.BrandName)
+		if cpuid.CPU.X64Level() < *level {
+			// Does os.Exit(1)
+			log.Fatalf("Microarchitecture level %d not supported. Max level is %d.", *level, cpuid.CPU.X64Level())
+		}
+		log.Printf("Microarchitecture level %d is supported. Max level is %d.", *level, cpuid.CPU.X64Level())
+		os.Exit(0)
+	}
 	if *js {
 		info := struct {
 			cpuid.CPUInfo