Update interop test xplat header (#19576)

Update interop test xplat header (#19576)

* Remove no-op #define logic for Interop tests
Remove single usage of #define for function remap

* Consolidate all uses of CoTaskMem* functions into new CoreClr* functions
for the Interop native tests.

* Remove unused functions and make formatting consistent.

* Remove '_cdecl' and replace all uses with '__cdecl'

* Convert '__stdcall' to 'STDMETHODCALLTYPE' for all non-calling convention dependent tests

* Replace 'CALLBACK' and 'NATIVEAPI' calling convention macros will well-defined 'STDMETHODCALLTYPE'