Adding support for multiple returns in compiled functions.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index b25fe413c96c40f69fa5a1e942bb3b2f9aa08644..7d44a1a0acc06ea3aea85d630e4cf783a6ffd662 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -304,8 +304,8 @@ class Linkage : public ZoneObject {
   }
 
   // Get the location where this function should place its return value.
-  LinkageLocation GetReturnLocation() const {
-    return incoming_->GetReturnLocation(0);
+  LinkageLocation GetReturnLocation(size_t index = 0) const {
+    return incoming_->GetReturnLocation(index);
   }
 
   // Get the machine type of this function's return value.