GcInfo: Report Struct containing one GcPointer correctly

GcInfo: Report Struct containing one GcPointer correctly

This change fixes a bug in the the GcInfo header,
where the ReturnKind is reported incorrectly for certain structs.

Structs that contain only one GCPointer or ByRef, can be returned
in a single register (Windows/Unix/OSX). The ReturnKind for these
struct-returns should be RT_Object/RT_ByRef, but they are currently
incorrectly reported as RT_Scalar. This change fixes the problem.

Fixes: dotnet/corefxdotnet/coreclr#11260

Commit migrated from https://github.com/dotnet/coreclr/commit/060e0c660decc4a694765bd1133fe1e5a59536be