[metal] support int64 (#7105)

Author: antinucleon

src/target/source/codegen_metal.cc

@@ -180,6 +180,9 @@ void CodeGenMetal::PrintType(DataType t, std::ostream& os) {  // NOLINT(*)
       case 32:
         os << "float";
         break;
+      case 64:
+        os << "double";
+        break;
       default:
         fail = true;
         break;
@@ -208,6 +211,9 @@ void CodeGenMetal::PrintType(DataType t, std::ostream& os) {  // NOLINT(*)
       case 32:
         os << "int";
         break;
+      case 64:
+        os << "long";
+        break;
       case 1:
         os << "bool";
         break;