[Wasm] Add missing return buffer argument in wasm import thunks

[kg]

Fixes errors like

Failed to construct WebAssembly module for Webcil image: {
  wasmPath: '/Users/kg/AppData/Local/Temp/wasm-ryujit-runner/sandbox/IL/test-module.wasm',
  errorMessage: 'WebAssembly.Module(): Compiling function #39 failed: not enough arguments on the stack for call_indirect (need 3, got 2) @+8418'
}
Unhandled exception. System.BadImageFormatException: Bad IL format. The format of the file '/Users/kg/AppData/Local/Temp/wasm-ryujit-runner/sandbox/IL/test-module.wasm' is invalid.

#39 WasmDelayLoadHelper->EagerImports->SignaturePointer_ReadyToRunHelper_DelayLoad_MethodCall(ImportSection:MethodImports,Kind:DelayLoadHelper,Sig:S16p) ( i32, i32, i32 ) -> none (75 byte(s))

cc @davidwrighton @AndyAyersMS

[kg]

Is this always going to be local 1 or will there be cases where it's 0? Statics?

[AndyAyersMS]

Order is:

• $sp if managed
• this if instance method
• retBuff if returning struct by-ref

[Copilot]

Pull request overview

This PR updates the WebAssembly import thunk emitter in the ReadyToRun AOT toolchain to account for signatures that require a hidden return-buffer (struct return) argument, preventing invalid wasm codegen at indirect call sites.

Changes:

• Detect struct-return signatures that require a hidden return buffer argument.
• Attempt to push the return buffer argument before the final call_indirect dispatch in the import thunk.