Crossgen2 fixes to enable composite build with shared framework (#34431)

Crossgen2 fixes to enable composite build with shared framework (#34431)

These changes involve targeted fixes to issues seen due to the new
build mode and generic signature encoding fixes identified in
my offline investigation with JanV. While I locally see the tests
passing with just a handful of failures, I still seem unable to
make the tests run in the lab; I keep investigating that but
I believe it's useful to merge this delta in to let us parallelize
follow-up efforts.

Thanks

Tomas