This property was only legitimately by the project which wants to
restore `crossgen`. Since that now comes from
Microsoft.NETCore.App (i.e. the shared framework) use
SharedFrameworkRid instead.
I also ensured that we passed UsePortableLinuxSharedFramework to
dotnet restore when restoring the tool projects, so we can use the
platform specific RID when non portable build.
Since Microsoft.NETCore.App 2.0 will only support 10.12 and later, the
RID in the runtime package has changed to osx.10.12. We need to update
so we pick up crossgen from the right place.
