Emit type layout check fixups (#34927)

Emit type layout check fixups (#34927)

* Emit type layout check fixups

When a ready-to-run compiled method uses value types from other version bubbles, emit a precode fixup to check at runtime that the type is still compatible (same size, alignment, GC layout).

Bring over `GCPointerMap*.cs` from CoreRT repo to get the compute the GC layout bytes that match what the runtime expects in the layout blob.

Simplify the behavior of the Crossgen test scripting. If tests opt out of crossgen by setting `<CrossGenTest>false</CrossGenTest>`, skip emitting the crossgen commands into the test run script. Also, stop setting the `RunCrossGen` and `RunCrossGen2` environment variables in the test script based on compile-time settings. These get set by runtest.py anyway. This set of tweaks allow for crossgen tests which we don't want to run the default crossgen commands on, and allow choosing either crossgen or crossgen2 as part of test execution.