Implemented return keyword

src/build/asm/Mnemonic.go

@@ -5,6 +5,7 @@ type Mnemonic uint8
 const (
 	NONE Mnemonic = iota
 	MOVE
+	RETURN
 	SYSCALL
 )
 
@@ -14,6 +15,9 @@ func (m Mnemonic) String() string {
 	case MOVE:
 		return "move"
 
+	case RETURN:
+		return "return"
+
 	case SYSCALL:
 		return "syscall"
 	}