[wasm][coreclr] Fix native calls to instance methods with 0 parameters (#121312)

The `nSizeOfArgStack` was calculated wrong, because the size of This
argument was not included. For cases with more than zero parameters it
is included in first arg offset.

It is different on wasm, because we don't use register for This argument
as all the other architectures.

It fixes the problem in
#121107 where the
`GetExceptionMessage` call ended without the `OBJECTREF throwable` on
the stack, resulting in random pointer instead.

---------

Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: radekdoulik

src/coreclr/vm/callingconvention.h

@@ -2190,6 +2190,15 @@ void ArgIteratorTemplate<ARGITERATOR_BASE>::ForceSigWalk()
     // Clear the iterator started flag
     m_dwFlags &= ~ITERATION_STARTED;
 
+#ifdef TARGET_WASM
+    if (this->NumFixedArgs() == 0)
+    {
+        // We have zero arguments, but we still need to reserve space for hidden arguments which are in registers on other architectures
+        // in non-zero argument cases, the offset is already included in first argument offset.
+        maxOffset += m_ofsStack;
+    }
+#endif // TARGET_WASM
+
     int nSizeOfArgStack = maxOffset - TransitionBlock::GetOffsetOfArgs();
 
 #if defined(TARGET_AMD64) && !defined(UNIX_AMD64_ABI)