Fix issues found in recent Int128 ABI change

[davidwrighton]

• Crossgen2 compiler was failing to detect that structs containing Int128 needed marshalling
• The contract we provide is that Int128 is aligned within a structure, but we do not provide guarantees around 128bit alignment itself, so the test needed updating for that.

Fixes#74318

[akoeplinger]

@davidwrighton I've seen this failing in 7.0-rc1 too, do you plan on backporting the fix?

[davidwrighton]

@akoeplinger I plan to fix this in 7.0, but not port to 7.0-rc1 unless there is some specific ask to do so.

[davidwrighton]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/2914908267

[akoeplinger]

That's fine by me, I just saw it failing in 7.0-rc1 as part of Test Monitor.