[Wasm RyuJIT] Conservative GC

[kg]

See also #121830 and #121651

We will initially run the GC in a conservative mode, where any refs or ref-shaped values on the stack are pinned. We need to ensure that the JIT spills any GC references temporarily stored in Wasm locals so that at any point where EH or GC can happen, they are in linear memory and visible to the GC.

(incomplete - work in progress)

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Closing as completed. cc @AndyAyersMS