[wasm][coreclr] Fix GetOffsetOfArgumentRegisters

[radekdoulik]

Fixes remaining issue mentioned in #120613

This fixes offset calculation of This and other special cased arguments.

Without that fix, we ended writing the this argument value to a wrong address.

[Copilot]

Pull Request Overview

This PR fixes offset calculation of special arguments (this pointer, return buffer, etc.) in the WASM target by introducing TARGET_REGISTER_SIZE to handle the difference between WASM's stack slot size and pointer size. On WASM, registers don't exist so arguments that would be in registers on other architectures are placed on the stack after the transition block, requiring different size calculations.

• Introduces TARGET_REGISTER_SIZE macro that uses INTERP_STACK_SLOT_SIZE for WASM and TARGET_POINTER_SIZE for other targets
• Updates GetOffsetOfArgumentRegisters() to handle WASM like AMD64 Windows ABI by placing arguments after the transition block
• Replaces all TARGET_POINTER_SIZE usage with TARGET_REGISTER_SIZE in offset calculations for special arguments

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[radekdoulik]

I wonder whether it would be cleaner to replace the parts (of the arg iterator and transition block) mentioning registers on wasm.

[jkotas]

I wonder whether it would be cleaner to replace the parts (of the arg iterator and transition block) mentioning registers on wasm.

I think it would be cleaner to decouple the arg iterator and wasm calling convention from the interpreter implementation details. I understand that this coupling is path of the least resistance at the moment, but I am not sure whether it is going to work well once we have AOT-compiled code in the picture.