(Re)Enable NativeVarargs for CoreCLR (#18373)

(Re)Enable NativeVarargs for CoreCLR (#18373)

* Enable NativeVarargs for CoreCLR

This will allow coreclr run programs which have native varargs.
It also re-enables the ArgIterator type for managed to managed native
vararg calls. This will allow the use of the __arglist keyword.

Limitations:

NYI statements have been added for all Unix Targets.

With this change __arglist (native varargs) will be supported, but not
yet tested on:

Amd64 Windows
x86 Windows
Arm32 Windows
Arm64 Windows

This change does not re-enable native vararg tests. This will be done
in a separate change.