CoreCLR: make UnwindNativeCode/UnwindManagedCode invalid values for exception marshaling

[rolfbjarne]

CoreCLR won't unwind native frames for managed exceptions (CoreCLR will abort), so we should make the "UnwindNativeCode" value for managed exception marshaling a build-time error. And for consistency we should also make the "UnwindManagedCode" value for Objective-C exception marshaling a build-time error as well.

[rolfbjarne]

This was fixed quite some time ago: 427c544.