GenASM: Use leave instead of mov and pop for returning from a function.

src/main/java/de/hsrm/compiler/Klang/asm/ASM.java

@@ -172,6 +172,10 @@ public class ASM {
     mnemonics.add(new Cqto());
   }
 
+  public void leave() {
+    mnemonics.add(new Leave());
+  }
+
   public void ret() {
     mnemonics.add(new Ret());
   }

src/main/java/de/hsrm/compiler/Klang/asm/mnemonics/Leave.java

@@ -0,0 +1,8 @@
+package de.hsrm.compiler.Klang.asm.mnemonics;
+
+public class Leave extends NoOperandMnemonic {
+    @Override
+    public String toAsm() {
+        return "leave";
+    }
+}

src/main/java/de/hsrm/compiler/Klang/visitors/GenASM.java

@@ -551,8 +551,7 @@ public class GenASM implements Visitor<Void> {
   @Override
   public Void visit(ReturnStatement e) {
     e.expression.welcome(this);
-    asm.mov("q", "%rbp", "%rsp");
-    asm.pop("q", "%rbp");
+    asm.leave();
     asm.ret();
     return null;
   }