Add PInvoke/Array tests (dotnet/coreclr#19266)

Add PInvoke/Array tests (dotnet/coreclr#19266)

* Add PInvoke/Array tests

* Fix compile warnings

* Get tests passing on Windows again.

* Change error_t type to work xplat.

* Change error_t in types.h as well

* Clean up build slightly.

* PR feedback and native test code refactoring.

* Clean up header.

* Add a xplat implementation of _itoa_s for our tests.

* CHAR -> char

* Make calling convention cdecl to avoid stdcall mangling.

* Make return test native code cdecl as well (missed it before).

* Fix bug in platformdefines _itoa_s

* Return early in TP_itoX_s when num == 0

Commit migrated from https://github.com/dotnet/coreclr/commit/f5a23fe0e3120e449ddce4b9c025305a786a2481