Support !JitDoOldStructRetyping on other platforms. (#35943)

Support !JitDoOldStructRetyping on  other platforms. (#35943)

* Add more test cases.

* Initialize `ReturnTypeDesc` when we keep struct types.

* Add a few const modifiers.

* Additional checks in `LowerRet`

* Support `return double(cnst int)`.

* Optimize `LowerRetStruct`: no need for bitcast when read from memory.

* Prepare `LowerNode` for store local and local field to multireg.

* Compile the new methods with FEATURE_MULTIREG_RET.

* Improve `LowerRetStructLclVar`.

Don't use bitcast if the source is in memory or has the same type.

* Extract `LowerStoreLocCommon`.

* Support 3, 5. 6, 7 bytes structs in `LowerCallStruct`.

Move call handling to the users.

* Disable `JitDoOldStructRetyping` for x86 and x64.

Windows  x64 was supported in a previous PR, this adds x86 (Windows and Linux) and x64 Unix.

* Fix suggestions.

* Disable by default for the merge.