Return a somewhat sane default value for L2 cache size if cpuid returned something unexpected

Fixes #1610, the KVM hypervisor on Google Chromebooks returning zero for CPUID  0x80000006, causing DYNAMIC_ARCH
builds of OpenBLAS to hang

kernel/setparam-ref.c

@@ -647,7 +647,9 @@ static int get_l2_size_old(void){
       return 6144;
     }
   }
-  return 0;
+//  return 0;
+fprintf (stderr,"OpenBLAS WARNING - could not determine the L2 cache size on this system, assuming 256k\n");
+return 256;
 }
 #endif