Avoid UB object reinterpret in builtin marshalling.

[MichalPetryka]

See #130279 (comment)

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?

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