[Wasm] Add missing return buffer argument in wasm import thunks (#128104)

For wasm import thunks with a return buffer argument, we weren't accounting for it and the result was an unbalanced stack.

Author: kg

src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/WasmImportThunk.cs

@@ -128,6 +128,7 @@ protected override void EmitCode(NodeFactory factory, ref Wasm.WasmEmitter instr
 
             int[] offsets = new int[methodSignature.Length];
             bool[] isIndirectStructArg = new bool[methodSignature.Length];
+            bool hasRetBuffArg = _wasmSignature.SignatureString[0] == 'S';
 
             int argIndex = 0;
             int argOffset;
@@ -195,6 +196,11 @@ protected override void EmitCode(NodeFactory factory, ref Wasm.WasmEmitter instr
                 wasmLocalIndex++;
             }
 
+            if (hasRetBuffArg)
+            {
+                wasmLocalIndex++;
+            }
+
             for (int i = 0; i < methodSignature.Length; i++)
             {
                 TypeDesc paramType = methodSignature[i];
@@ -302,6 +308,13 @@ protected override void EmitCode(NodeFactory factory, ref Wasm.WasmEmitter instr
                 wasmLocalIndex++;
             }
 
+            // Pass return buffer argument if needed
+            if (hasRetBuffArg)
+            {
+                expressions.Add(Local.Get(wasmLocalIndex));
+                wasmLocalIndex++;
+            }
+
             for (int i = 0; i < methodSignature.Length; i++)
             {
                 TypeDesc paramType = methodSignature[i];