Avoid UB object reinterpret in builtin marshalling.

See https://github.com/dotnet/runtime/pull/130279#discussion_r3539930677

The safest solution would be to use a pinned local for the object reference (those are valid in IL) and then `conv.u` the object reference and add data offset to the pointer.

cc @MichalStrehovsky @AaronRobinsonMSFT do you maybe want to use copilot on this?

Start with the linked pull request discussion and trace the builtin marshalling implementation it discusses. Replace the object reinterpretation approach with the proposed pinned local, `conv.u`, and data offset handling; done means the marshalling path no longer relies on undefined behavior.

Written by the indexing model from the issue text.