Implemented return values

src/build/asm/Instructions.go

@@ -66,6 +66,10 @@ func (a *Assembler) Jump(name string) {
 
 // Return returns back to the caller.
 func (a *Assembler) Return() {
+	if len(a.Instructions) > 0 && a.Instructions[len(a.Instructions)-1].Mnemonic == RETURN {
+		return
+	}
+
 	a.Instructions = append(a.Instructions, Instruction{Mnemonic: RETURN})
 }