Vector abi tests and reverse pinvoke fixes (#32034)

Vector abi tests and reverse pinvoke fixes (#32034)

- Fix issues noted in returning structures from reverse p/invokes

* Add tests for marshalling Vector2/3/4
- Anticipated work on Windows Amd64 calling conventions may affect this, so this needs testing
* Updates to add return struct marshalling tests/fixes
- Adjust some reverse p/invoke struct marshalling tests to add coverage for returning structures
  - Coverage includes a 3 byte, 4 byte struct, 8 byte struct, and a large struct
  - Note: There is a code in mlinfo.cpp which explicitly makes support for returning 3, 5, 6, and 7 on X86 follow the wrong calling convention by rounding the size to a 4 byte boundary. I've left this unfortunate detail in place.
* Re-encode ReversePInvokeTest.cs to utf8 to reduce future diffing burden