Add native vararg test (#17707)

Add native vararg test (#17707)

* Add native vararg testing

This is a desktop test. It will be excluded for coreclr; however, decided it is worth adding
to the test tree in case we ever need the coverage.

* Fix several errors, and address feedback

* Split long lines

* Update vararg test

* Fix excludes

* Refernce private_corelib

* Correctly build vararg test

* Fix incorrect addition

* Fix build break unix

* Fix native build on unix

* Fix entry point

* Correctly pass doubles instead of floats

* Correct vararg test, pass double to pinvoke methods

* Remove printf tests

* Fix cmake for varargnative.c

* Fix cdel calling convention

* Correctly pass success back to the main method

* Explicitely undef cdecl

* Fix some excludes and exclude the new test on x86

* Add unix exclude

* Disable echo_short and correct header